We have to serve child first or old people when they dining with a group of people

What is the culture background, if you are German the older custom is to always serve the adult men first.

Many customs serve the older first, but then many serve the kids, get them out of the way to a children's table and then adults

I was raised using the rule that you start serving with the senior-most female guest at the table - or it it's all family then the senior-most female. Then continue in order with the rest of the females (not by order of age, but by where they are sitting, starting to the left of the person you served first) - including girls as well as women. Then serve the men and boys - pick one to start with (senior male guest, or senior person) and continue in order around the table. Nobody starts eating until all have been served at the table, unless the host or hostess indicates otherwise.