ACK NO Cornstarch breeds more rash.
I had 7 kids and EVERY one of thenm had a super sensitive diaper rash butt up to age 3-4. After lots of trail and error and questions to Drs. we finally found a MIRACLE. It's called CALDESENE and it's a powder in a pink plastic bottle. They sell it in places like Walmart or CVS . Just sprinkle some on each diaper change and it will help SOOOOOOOOOOOOOO Much.
Corn starch and Talc are no good. Did you know the critters causing most rashes actually FEED off that stuff and breed. It's like FOOD to them.
Caldesene is like 6 bucks a bottle but WORTH every lickin cent!!! One time we found it at a dollar store and bought EVERY bottle they had for $1 a bottle.
I wouldn't worry about your 3 year old daughter. I'm sure you could do homemade costume SO EASY. She will probably be wearing a coat so why not just do the good old CLOWM. use lipsticks and eyeshadows to clown her up and just put her hair up crazy in barrettes or bands with maybe strips of colorful old material tied in. or even some yarn tied to pigtails!!!