"I wish I had said something," said Beltz, 39, who added that she didn't because she was still a new mom and that today her reaction would likely be different.
Today Beltz is a member of a community message board in her Chicago neighborhood that reports on nannies and caretakers who are neglectful on the playground.
"People need to be aware," said Beltz. "When anything happens at the park the thread on the site gets longer and longer parents will watchdog and try to find the parent whose nanny [is misbehaving]."
Bailey said online message boards like Beltz's can be helpful for mothers who are concerned about their children's caretakers.
"I think that these sites are actually a good idea because moms can't be everywhere at once, so whatever moms can do to support each other is a good idea," said Bailey.
Not all mothers believe it's appropriate to comment on other's parenting. Leah Klein, a mother of two toddlers from Boston, said she does not agree with parents who intervene in other people's lives.
"I don't think its appropriate," said Klein. "More often than not, a passby does not have the big picture and how each family manages parent-child conflict or interventions is cultural as well as family specific."
Klein added that if it was a friend who she felt was mis-treating her child she'd be more inclined to speak up.
"If it is a friend or someone you know quite well, see if you can talk with the parents about how they are doing to get at the root of the problem and then, at a later time, maybe lend a book or forward a link as a resource to help them," she said.
"Don't overdo it though," said Klein.
Ilina Ewen, a mother with two children who blogs for the Deep South Mom's Blog from her home in Raleigh, N.C., said that barring an extreme situation she wouldn't intervene in another parent's situation either.
"I'd never come out and say, 'God, you're mistreating your child,'" said Ewen.
Ewen said that she often uses the tactic of distracting the child with a snack or even offering that she watch the parent's child so that the tension can be broken.
"I've said before to another mother, 'I've so been in your shoes -- if you want to take a break I can watch your daughter,'" said Ewen.
While Ewen does say that she wishes parents weren't so judgmental of others she admits that she too is guilty of occasionally eyeing other mothers and fathers on the playground.
"I think we do tend to judge, but it's often because we see ourselves in whatever it is we're witnessing."