One quick cheap and easy possibility that I have used when I first got my trike, I used standard road clips and a velcro strap running from them around the back of my shoe and back to the otherside of the clip, they were very effective and much harder than clipless to pull out of and limited sideways movement severely.
Originally Posted by MichaelW
You can create a heel clip, much like a toe clip from bent metal. Bolt it onto the rear face of the pedal.
I was thinking along similar lines but with a solid plate of thin metal with the edges bent up so the heel can't twist.
If you wanted to, it may be easier to make a low sided metal box that her foot will just fit in and bolt it to the face of the pedal then, that way it would also help keep the front of the foot inline.
May even be worth a look in the local xmart and see if they have a plastic food container the right size and cut down the sides as required, if needed you could slit the bottom and put a velcro strap or 2 over the foot.