what about a visual cue? like a bright pink sticky note on her toe - she has to make sure she can always see it.
also, you can stick a dollar under the calf and ask her to keep it there.
other balance exercises that I used to have my students do included reaching down and touching their toes, put feet in/out of stirrups, "around the world" where the rider turns completely around on the saddle (supervision and helping hands necessary!), take arms out to the side and do circles with them, also, have her stand on the ground with her hands on her hips and "pretend post" - you can then have her do it on a stool: the first time have her sit with her legs far back under her - and ask her to stand, then the second time let her bring her feet out in front of her, and have her stand again