I see a bunch of kids in the office for orthotics and many of them see me frequently for new top covers. The most frequent reason, is: the orthotics got wet and the cover either came off or smells or looks poor. I was also getting similar complaints with the rearfoot post becoming saturated. Typically this involves a puddle at recess that got played in and the shoes are soaking along with the socks and the orthotic device. As everything dried out that evening, parent and child observed damage.
I decided to try out a completely waterproof rx and parents are pretty happy!
Placing kids in direct milled polypropylene orthotics is a great idea for the great feature of being water proof. Another option is the vacuum formed polypropylene shell with poly rearfoot post. As long as you can do what you need to do without a needed sweet spot, forefoot accommodations, or top cover, you are set!