What can cause foot pain in young children?

"growing pains"? . Sometimes, children get pain in bones that have not yet closed or fully formed. I.E. Pain to the heel in a child is often a condition called calcaneal apophysitis or sever's disease. It is a benign problem and usually self limiting. Rest and ice help it. (there are other causes this being a more common one.).
So many causes... Too many causes to list, but here are some more common ones: plantar fasciitis, bursitis, calcaneal apophysitis, trauma (contusion, sprain, or fracture), symptomatic flatfeet, bone or soft tissue tumor....I apologize for the list, but the question was very broad....Probably better to see a podiatrist in person where he can take a full history a narrow down the possibilities. Good luck!