I don’t think you should visit them to check if the child is ok.

I meant visit them or invite them over or go places with them to get to know them better.

People who don’t know my cousins kids well wouldn’t understand why one of them always sits by the parent at big parties. But we know why because we know them well. If you don’t know them well, I suggest you try to get to know them better

You also mentioned that they might behave normal in their own small family. That's important point

When you observe them you might come to a conclusion that she isn’t comfortable at large parties or her kid isn’t but there are ok in other settings. Could be she is anxious or the kid is (not because she is not good looking, lots of good looking people are anxious at parties). If you get to know her better you might have a better answer about some of the concerning behaviors