First of all, welcome to Roadfood!
Even though I know Pittsburgh restaurants fairly well, since I don’t have children, I’m not very good at saying if they are kid friendly or not. But, both De Luca’s and Original Hot Dog Shop are local institutions and are well worth the visit. For breakfast
One place I would suggest would be Klavon’s, a lovely old fashioned soda fountain that I imagine a five year old would enjoy.
If you have a specific type of food you would like to try, please let me know and I will be happy to have some other recommendations for you.