Many of my students have been forgetting some of the basics around technique. Here's a handy chart that you can post by the piano or on the first page of your music notebook. Probably the most important one I'm finding is sitting the proper distance away from the piano. Many kids like to sit almost with their bellies touching the piano. This makes it so much harder for their fingers to be in the right shape to play well. You should be sitting so that your forearms are about level with the floor, elbows bent and shoulders not hunched or lifted.
Curling the fingers can take some time to remember for the youngest students. I usually tolerate the flat-fingers for a while until they get a few pieces memorized.