Sorry but running around after DC letting them change their mind up to the night before is weak parenting.
You ask them when the letter comes out,explain the budget and resources constraint and then explain that once they have decided that is it.
No way would I be running around at the last minute because my little emperor had changed his/her mind at the last minute. It's a life lesson you make your mind up and stick with it particularly when somebody is helping you.
wow your house must be full of empathy, fun and joy.
Hope you are teaching your child to be as judgemental and bitchy, so they can pull their judgy pants on easily.
could there possibly be any space for an insecure child who doesn't want to look silly in a costume, and once they have heard what everyone else is going as, get cold feet about their choice, and will probably have a panic meltdown?
Or even, that the school's first letter just said dress up and the second one decided there was a theme?
Or just responding to please, please, please Mummy? I am, after all, not a policewoman, and being nice is allowed as a mother sometimes I believe.
My house is not run as a gestapo headquarters. Pick your battles and all that.
My personal favourite was dd1 in year 6, who went in as the wardrobe from Lion, witch and wardrobe. Inspired by me wrestling a large cardboard box into the recycling.
The only slight fly in the ointment was she told another girl the day before and that girl also came in as a wardrobe. (but dd's was better)