How could I help heal a pressure sore on a toddler's abdomen caused by diaper rubbing?

Aquaphor. Apply aquaphor or vaseline or zinc oxide cream to the sore. If it looks infected apply a topical antibiotic ointment such as bactroban (mupirocin) (mupiricin). Avoid rubbing the area. Change the diaper more frequently and don't wrap too tightly next to the skin. Let the area air out longer if possible.
See you peds. Diaper rubbing requires that you learn another way to care for the child. Discuss with your pediatrician.
Diaper injury. Try to keep the diaper off as much as possible or practical. Lotion the skin as frequently as you can. Change and or try a different brand of diaper that rises at a different height. There are skin protectants that can be used to protect the skin but you would need to check a smaller area to make sure there is no reaction. Protecting with emollient like zinc would also provide a barrier too.