Is it normal i regret not telling a little girl that i liked her costume?

It is unfortunate that we live in a time when men have to be very careful about any sort of interactions with children, but that's the reality.

I think that I wouldn't have said anything either if it was one of those awkward things where you have to share a table with others but they're clearly trying to pretend that you're not really there, talking amongst themselves and avoiding even eye contact. But if the adults of the family and I were chatting while we ate, I most likely would have something, although what, to whom and when depends on various things.

You don't state the age of the girl, but if she was young enough that it was obvious that her outfit was something the parents had sorted out, I would have looked the adults in the eye and said something complimentary, and then said something directly to the child. It would be a bit trickier if the girl was pre-pubescent or older. If the chat with the parents was going well and the girl herself was getting involved in the conversation, I'm sure I would have complimented her in passing. If she seemed shy or awkward, I wouldn't have said anything until either they or I finished eating and got up from the table. That would relieve her of any pressure to respond, and the fact that you were in the process of going your separate ways would remove any potential creepiness from the situation.

Life happens in real time not hindsight

Why? Just tell her you like the costume. It's not like by saying that she will die

What comic con did you go to? The big one in San Diego isn't until November.

A good rule of thumb is to divide your age by 2 and add 7