I also make cakes and live in South Florida with RIDICUOUS heat. Crisco makes my arteries clog just thinking about it, but it does help the frosting hold up in this heat & humidity.
I use this recipe, but you may want to use butter-flavored crisco instead of regular crisco.
1/2 c butter
1/2 c crisco (don't use store brand)
2 tsp pure vanilla extract
4-6 cups of confectioner's sugar (depends on the humitity at the time you make it)
2 tbsp corn starch
2 tbsp milk
cream butter, crisco, and vanilla.
sift 4 cups of the confectioner's sugar and corn starch and add to butter/crisco mixture.
Add milk and additional sifted confectioner's sugar until you reach desired consistency. you don't have to add all 2 tbsp of milk. add just a bit at a time until you get what you are looking for.
Tip: Try to keep it medium stiff, not really floofy and it will stay well for most of the bake sale. She can add it to the top of the "cone cakes" with a pastry bag and a large, round tip for a really pretty, clean effect. You will want to keep them in the shade once you get there to keep them looking nice for the maximum amount of time.
best of luck to your daughter and her squad!