My girl seemed to have an instinct to be gentle with kids. I brought her out and around the city and parents would just bring their kids over to pet her. She is very gentle with them, licks them and lets them hug her. Its a cute thing to watch.
Any little family members nieces nephews cousins can visit, neighborhood kids. Walks in the morning when kids are at bus, playground areas. One time we actually brought Max into toysrus when he was a pup. it was at night and quiet. I'm not sure if you are allowed to do this but the employees did not say anything. It was a really crazy cold winter so I just took our pup out to a new place whenever we could.
I would say...just teach your pup manners (no jumping into people or chasing people while they are running) they kinda...know they boundaries too but if you have nieces whatever and you want to try to do some training while they are around hahaha (i did it once) let the kids know how to act when your dog is doing something that is not proper and correct your dog direct his/her attention to another thing. Good luck!
After Samuel's puppy 2 shots I took Samuel with me to watch my little cousin play baseball. He would have no less than at least 5 kids wanting to pet him at all times. Sticky fingers and all. He loved the attention. So now at a little over a year old we may have other issues but never with any kids. He is so respectful and never jumps on them. He is used to little kids running around and screaming and approaching him all crazy. I have friends with young children and they trust Sam to be, "Gentle" that is his command when there are children around, at all times.