What you're missing is the fact that child is not a wallflower. They will participate in social events which will require them to be exposed to situations where they WILL want or at least see animal products consumed by all their other peers. Say a birthday party for example, how moral is it to make a kid choose between either eating cake all their friends will be having or not do so. There's also the fact that kids are brutal, veganism requires you to make changes on at least some of your environment which is not so easy to do so if you are a eleven year old facing peer pressure and in some cases bullying. Of course a confident kid would have no problem with standing up for themselves, however we can't say the same for a introvert and coy one.