What would you do in this situation?

Leave your kid at home. Even if you fib to the bride and groom and say you can't get a sitter, you know that your kid isn't welcome. Either attend without him or one of you can stay home with your kid while the other spouse attends. Figure out who's attending before you send back the response card. I don't know why people take offense when kids are omitted from weddings, you should see it as a chance to have a nice nite out as adults.