Although it's not school aged children in diapers there are some photos of kids in diapers on the website at this link below.
I've sometimes been able to find some older kids in diapers in pics by searching google images and bing images as well.
I also wish I knew of other websites where older school aged children in diapers could be found.
Here's a link to one photo of 2 older kids wearing diapers but it's not the best photo.
Hopefully someone chimes in and can let us know I would love to know as well.