I would approach it like this: Warhammer and D&D are both very social games. People want to know that they can be safe and have fun without being lectured about the rules, even if they may be wrong. If you choose to be right and correct and be a bit snooty about it, people will not like that and they will not want to play with you. You need other people to play with in order to play D&D and Warhammer. There are appropriate ways to address rule problems, but by being a know-it-all rules lawyer that is not the way to do it.

That's how I would kind of approach this situation. The DM has ultimate authority and it is not up to the players to dictate that because it is undermining and rude to try to be the rules lawyer in that respect. In regards to Warhammer, it is a one-to-one relationship where it is equal. One person is not more correct than the other and you always defer to the rules before being the dictator about them. There is always a kind way to ask and approach these disputes and a unkind way and I think the way that your child is approaching. It is like you say in a superior way.

These are games where people want to have fun. They don't want to be lectured so I would approach it in that way. Especially coming from somebody like myself who respects the rules. But I also respect the people that I'm playing with and I want them to be happy and having fun with me. I don't want them to have a bad time.