Question: Is it a good idea to lift weights with my son or daughter, and if so, at what age should he/she start?
Answer: I think it's a great idea to exercise with your son or daughter. It gets them exposed to exercise as a lifelong healthy thing. And at what age should they start really depends on where their maturity level is and your comfort level with teaching them how to exercise.
I think when it comes to weight training we get a little bit scared of getting our young athletes into weights too soon. It's a valid concern because sometimes our athletes aren't ready for that. But in the hands of a trained strength and conditioning professional, getting athletes into a resistance training mode early can be very safe and very advantageous to them in their growth. Working with body weight exercises to begin is definitely the way to go to get technique down.
So, if your working with your young son or daughter don't worry about putting any weight on, don't worry about resistance training the way you would consider to be resistance training. Work on getting them to learn to squat with their body weight. Work on learning to get them to push up their body weight. Work on pull-ups with them. Those three main exercises are going to go a long way in teaching them proper techniques and mechanics and improving their strength overall.