Mom, step-dad open up about daughter's tragic murder by her own father

One month after their daughter Kayden's death, Kathy Sherlock and Brian Sherlock, are opening up to "Nightline" in their first joint television interview about their unimaginable loss.
9:57 | 09/06/18

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Transcript for Mom, step-dad open up about daughter's tragic murder by her own father
smile reporter:ht song seeme to be yden Mancuso's personal anthem. ??? I might one ake an elosion ??? E their kid's the most amazing person on the planet,ut this little girl was diet, she was special. Everyone wanted beer best friend. T 7 yearsldhat bright lightfaded. Her brant life cut short in a flash. I still don't feel like it's real. Ieel like I going to ke anup this awful nightmare -- but ow that I'no of a raging custo battle. En killed by her own biological father, Jeffrey murder, kayd's mom and cassie a Brian Sherlock, speakout against a family court system they say failedkayden by notroperly king intoccountjeffrey's olent history Andy sayse her repeated crfor lp. Do you feel lrdaughter's death was preventible? Absolutely, oh my D, absolutely. 100%. Every S thing thatould vetopped it it jdi Reporter: With THRs and a new the sherlo coul'tetter. But therwas somebody else in the pie. Kayden's bgical far Jeffrey USO, whoathy met one summer onseyshore. T was a fling? Yeah,asically. Reporter: Ted but T moved inogether wn Kathy found out she was nt I taught that' what I had to do, to try ka to give her two pas. R despite the between mom and da site Alison says T father/dter bond was strong. She WOU always say how S loved him. My brothers 5'6". And she whis like, she jt thought was so tl and so strong. Hi, ikayden. His Ster says was troubled since childhoo issues made worse, she sa, B punching teachers E head with a pool stick,e D 17 staples head or somet like that How much would he if I have a beer with M, I'd ob my second, H ob H 13th beer. It was that drinking. Between the dnking a hits to the head, you know he suffered from se brain damage caused him progressively get menlly ill. Reporter: Kat soon disced that Kayden's dad H a viont streak and a criminal ludingultipleassault chargeseveral states. And he hadtrangled me. I remember the te er felt like I was going te in E clutches of his arms Reportee ago the final straw. Jeffrey was charged with afteret another bar fight. Ime he bit off part of a man's ear. I left, literallyke my house in a U-Haul on a Monday said I gettinghe hell out of here. Repornt desp fearing for her own safety she never ined jeffy would physicall harm their prdaughter, so she establ iormal cdy schedule with Jeffrey while H gaveer moneyor child support. Loving towarr. I mean, they did a lot of it P T only G thing he had in his life. Rter: Asjeffrey's life seemed toe unraveling, Kathy was movinon. She fell in love with brn. We create as marrge -- was love at first sight for thatit girl. You fellove with both of them? Yeah, it was defely B of Rorter: Las year Jeffrey parcustody. Jeffrey repre in court. In front of the and the sherlocksay he went on the Jeff attacking K L these accusations. It wasn't procedure. What was the judge'se in Y view? He was just , I'll let .oh, that's okay. Reporter: They say jefey led out at Kathy outside of T o,ng the police onover and over again. He hey say, gre violent wnher in front of ka. What didl you about what she physically witnessed? Tt daddyhingia mom, she called her,n bed, crying. Or the first Alison admitted Kayden admitted to being of her dad. We asked her, didou think daddy was going to hurt you? She said yes. Ortle over a ar into the custody battle Kayden's desperate mom filed a reg order. Her lawyer never fed a -term one for ka. Everything. The police are like, E's not rting you. Reporter: K gathered tnesses to are her custody case, iding Kayden's teacher, cipal, a soccer D Smith, who witnessed a confrontationween two dads after Jeffrey brought kaydenatohe final game. I sat and watched Jeff get upon Brian's face. And he was- look like was trying to Provo into a fig switch went off in my that he needed pfessiona he He never had aracter witnesses testify O his half. Yoknow, not one person in his say, he's areatfather would neverch Reporter: Ihee of ting evideest Jeffrey and calls intervenon, Kathy feels as I E jud still didn't take her seriou He felt that I was just - crazy M. A crazy mom. He oed me a meal health evaluate. So at the that case, on the 28thhe's like, I think you both need help, think you both need Reporteathy was deemed by the psychiatricvaluator T have anxiety. But Jeffrey was diagnosed with major pressive moderate wh anxious disorder, and identified with narcisic an anti-socia personality aits. Hey said hhad no remorse, he had absolutely no SP for any of his on Reporter: Ato evaluator recommen S visits B unsuised only if he received mental health treatment. But while the judgedvised jersey to gehelp, he him unsupervised visitsh ndatory mental health treatment. There mig wonder, why follow court Ders if you feel like your daughter's in that grave nger? Treimes when I J said biggesttas I should have gone cont or jail. Reporter: Kathy says the judges decisionould have deadly coeq just one monthag Brian discovered theunimaginable. Got work. I turned left. And she was just laying the layinglittle girl. I knew she was dead. I mean, how could heo to yr own 7-ye-old. Whatith me what her lasten were. And D rot in hell. For what he DI Tig story tonight, the unfolding investigation into of grief - Reporter: Kayden's death rocked the communityith Famy and friends Askin past and mental instability. I feel in thicase it would have jusen one persoto lien to me, one person do something. Should have put visits, supervised. Even if they wr Faly member. Un he could truly evaluated, help, put on medicicommitted, whatever need te done. Reporter: An onlietition calling for the removal of the judge quickly gai popularity, with ove0,000 signatures. And T frankone of suppo center for child advocates says famiourt isught with contentious nd judges are oft confined by the law. It's not uncomm a parent with a crinal history, including crimesf violence to be awarded unsupervisedstody. Whennk of the nbers of person in this country who have criminal recos,lu for violent behavior, wld have lotsor in fosterar D lots more kids not seeing their parent. Reporter: Kayden's family believes the systeds T GE. This week ithhe goveor's office to push forwdkayden's law. Pteis right versus he essentially, his right to be a parent was more soortant an her right to essentially LI. Reporter: In a atement, the judge's offices court records do notport theerlocks' comments about the judicial proceedings or presiding judge, addinge stngly encou parents to sk help, not for their owpersonal benefits but for Kayden'so. The fact that I'm N to see her again -- I hate it. I haten't know how to go on. ??? I might only one match Bui can make an explosion ??? Repr:erer's hope that kaen's, like her beloved song willave an im. Itays in "The fso I M only one match but I caused anexplosion. Pefully she causesomt of explosion that this doesn't ppen to soone else. Give me a S

This transcript has been automatically generated and may not be 100% accurate.

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