Making a meringue buttercream frosting is tricky but well worth the effort. It has good stability for warmer weather.

Preppy Kitchen has a recipe for Swiss Meringue Buttercream. It is delicious enough that you will want to eat the whole bowl of frosting in one sitting, lol.

Be warned - don't use the premium European-style butter when making this frosting. It's too high fat. Use regular butter.

There's a bit of a learning curve for this style of frosting. I suggest a dry run before making it for an important event. But again, careful, or you'll find yourself eating this frosting straight out of the bowl with zero willpower. It's that delicious.

Ask neighbors or a Buy Nothing to borrow a cupcake carrier and cooler

I’d scrap the cupcakes and bring ice pops in a cooler!

They should not sit out. Put them in a cooler with ice.

The flour frosting is often called “ermine” frosting but I’m not convinced it would hold up any better in 90 degree heat.

OP you can use any box, a shoebox, whatever. Put the cupcakes in so they’re snug, fill any extra space with crumpled parchment. Put lollipop sticks or skewers in some of the cupcakes cut to just shorter than the top of the box. Or all of them, if it’s easy enough. They’ll be fine.

Cooked frosting with flour.

Not kidding:

https://tastykitchen.com/blog/2010/03/a-tasty-recipe-thats-the-best-frosting-ive-ever-had/

I've made this a few times and greatly prefer it to buttercream. The cooked frosting isn't as rich and is more stable.

Any cooked frosting or marshamallow frosting

I wish I were friends with the posters talking about the relative stability of different variations of buttercream. I love to bake, but don’t know a soul in real life who has any idea there are variations on buttercream!

Swiss or Italian meringue buttercream is more stable than American buttercream + a cooler to transport and store until the kids are ready to enjoy.

Cream cheese frosting or vegetable shortening instead of butter. It all depends on the melting temp. Italian buttercream will melt faster than vegetable shortening frosting or cream cheese because butter melts at a lower temp than either.

https://www.kingarthurbaking.com/blog/2018/08/17/best-cake-frosting-hot-weather

Or you could go fondant, but does anyone like the way it tastes? The hot weather icing conundrum explains why wedding cakes are often blech tasting compared to one with a real buttercream.

You need meringue powder- 1 TB for every pound of confectioners sugar. Add it with your first round of sugar during mixing.

Thanks! I was hoping to stay away from shortening (I don’t like the taste) and these are good options. I suppose I could rig something up to keep them cool in a cooler. I don’t have a cupcake Tupperware or anything to put in the cooler - will have to improvise.

I’ve never tried Italian buttercream but maybe I’ll give it a go.

Italian buttercream is super stable. It’s not as sweet as regular buttercream however.

You can use shortening, but a far better option would be a cooler.

Put the refrigerated cupcakes in a box or whatever (I’d go on Amazon and get some plastic cupcake boxes with hinge lids), then wrap the box really well with plastic wrap. Put that in the cooler (use ice packs instead of ice). Leave it alone, try not to open the lid.

When it’s 45 min before cupcake time, take the box out of the cooler and leave it wrapped! You want condensation from the initial temperature contrast to form on the outside of the plastic wrap. Leave them for 30 min. Then you can unwrap the cupcakes and put them out for eating.

I am bringing cupcakes to a kids outdoor event next week and the forecast is for 90 degrees. The cupcakes will have to sit out for an hour or two. I usually make buttercream frosting with just butter, powdered sugar, and vanilla. But that does slump in the heat. Does anyone know of a way of stabilizing it a bit? Any other recipes that people like?