One thing we have found with some children is they find it so relaxing sitting in the recumbent position that they can forget about keeping their feet up. (warm sun, relaxed and birds singing, it’s not un-heard-of to find a young one asleep at the wheel.)
There is a fairly simple and virtually pain free solution, which is to have simple pedal straps. You can also use bicycle cleated shoes and special receptive pedals – but this can be a little more expensive as young feet tend to grow quickly. Simpler to use shoes you already have as the straps are adjustable to many different shoe sizes.