I insulted a child who was wearing a costume, is this bad?
Why would a child be wearing a costume? Is this something you are serious about?
Many children want to play make-believe.
I can see making a mistake like that seems harmless for the most part given that you thought the child was wearing a costume. Regardless it's rude to make harsh observations about someone's attire, if you view something you don't like you should try to keep it to yourself. Insulting a kids attire could result in the parents getting mad at you and then a fight breaking out.
It was just a dumb kid and no big deal.
You should go over to the child and apologize, and make it fun. The child will be so upset for the rest of it's life if you don't. Would you want that on your conscience?