Instead of focusing on what they're doing wrong, I'd try to focus on what you actually want them to do. Maybe that will help. I know that you work with kids, and I bet it's more difficult to communicate with small children.
I go with the age old wisdom that criticism is best delivered between two compliments. So, I start with something positive, then I deliver my correction, then I end with something positive. That's what works for me.
Ugh, that's a tough one. It depends on the person I guess. Some people really take it too personally though. I would handle the situation really delicately and let them know what they need to improve in the kindest way possible.