I think children's shoes are especially hard. If your daughter's feet measure a size twelve, but they're too big inside for her feet right now, you might try a little "padding" until her feet get just that tad bigger. Or it could be that she simply has narrow heels.
See if the heels are or aren't the only problem. If all her foot is really sliding around inside the shoes, you could get some inexpensive insoles and cut them down to fit inside the size 12s, so her foot will have a little more security.
For grownup shoes, there are cushy inserts that stick to the back (inside) of shoes to make them more comfortable. You could try making something like that to take up some of that extra space in the heel. See if there is any moleskin at the store, in their foot care department. It's often cut into pieces and used on places where a shoe might rub a blister. I used to use it in narrow strips inside the backs of my daughters' shoes. Sometimes I would use several strips, one on top of the other. Sometimes it would work beautifully; sometimes the moleskin wouldn't stay in the shoe (I had to experiment to get it to do what I wanted it to); sometimes the adhesive was a little hard to get off after the cushioning wasn't needed. So this isn't foolproof, but it's worth a try, and you're not out much money if you're not satisfied.