So, the other night, I walk into Jack's room and see this in progress.
Steve: Come on, buddy. You can't wear your Batman costume out to dinner. Let's put on a nice sweater and a pair of pants.*
Jack: WAAAAHH! IWANTTOBEBATMANANDDADDYWON'TLETMEBEBATMANANDIDOWANTTOBEBATMAN!
Steve: Jack, if you act like this, we're not going to be able to go out for pizza at all. Remember, nice guys get nice things.
Me: Steve, tag me in and go take a break.
Steve: If he's going to be like this...
Me: I know, but let me try.
Me: Jack, see this t-shirt? It's a Batman shirt, just like the one in your costume.
Jack: But I want to wear my costume!
Me: What you could do, Batman, is wear this shirt under the sweater that Daddy picked out.
Jack: I want to be Batman!
Me: Batman, think about it. What if the Joker is out at the restaurant eating pizza. If you show up in your full-on Batman costume, he'll know it's you right away and he'll run off. This way, you can go in disguise and can catch him or any other bad guys who might be in the mood for pizza tonight.
Jack: Okay, Mommy.
Me: I'm Batgirl.
*for the record, I probably would have just let him wear the costume to dinner, becuase who cares? But it's more important to present a unified front if we want Jack to respect parental authority/if I don't want to sleep in the guest room