That's a hard one, you don't want to be rude to them either. Future reference, if something like that happens again just try turning the subject back to what it was before (birthday boy)and don't answer any more questions about yoy. (I know it might seem rude but just talk about the subject you are there for) chances anonymity the little boy paid zero attention to that and had a blast! Maybe you and fiance can take him out just the three of you for ice cream or something