Needing advice for my boyfriend

So my boyfriend asked me for advice, but I don't have kids so idk why he asked me. He has two kids 6 year old girl 4 year old boy. They both like to dress themselves and he's been begging his son to take off his cowboy costume he wore on Halloween but he's not budging. They are always late because they can never agree on what to where. His daughter wants to wear her princess dress everywhere or pajamas or something like that. His son just wants to wear his cowboy costume. What do you do when you can't agree with your kids on what to wear lol?