The tea party is an hour. I don't think I would send a child who isn't potty trained. If he has to go in the middle of the party there are only 3 CM's there with about 14 kids. It is pretty fast-paced.
As to a boy going, my youngest DS was 3 and he went with his older DS who was 8 last year. They loved it! Couldn't wait to show us the pictures and tell us all about the fun they had. Definitely let the boys join in!