The idea of having a foot stool for young children (whose feet can't reach the floor when sitting at the piano) is that they can have a better balance and feel more grounded with their feet secure and stable on a flat surface (i.e. the foot stool) while playing the piano. Without the foot stool, they may be unconsciously swinging their feet in the air (that could become a distraction to some kids) or unconsciously sitting on the whole bench (so that they feel secure that they won't fall off) or sitting on just a little bit of the bench (so they can reach the floor) ...either way is not good because that means they are either sitting too close to or too far from the piano, which does not allow students to move freely. The correct way is that we should sit on the front half of the bench while having a bit of our knees underneath the keyboard, and if our feet can't reach the ground after sitting in this way then a foot stool is needed.
Getting a proper foot stool like this with pedals can be quite expensive for students (about AU$150~200), an alternative option is to get a normal foot stool from IKEA or other variety stores. It'd usually cost AU$10-20, and can serve the basic purpose of keeping the student's feet stable on a flat surface while playing the piano. It'll definitely make a big difference even with such a little tool. You should consider getting one for your young little one if you haven't yet thought about it.