My 7 year old has a bump on her foot and it feels really hard but it doesn't cause any pain, what do u suppose it might be?

FOOT BUMP. Sounds like a wart. See a dermatologist or pediatrician.
Wart. This is most likley a wart. It only hurts if it is in a part of the food where pressure is applied. If it is in the arch it may not cause pain. They will resolve eventually but hard to say when. You can see your doctor to get a visual diagnosis and decide if you want to do anything about it.
Skin or bone? Depends on the location and "feel" of the bump. Could be anything from a wart to scar tissue to a benign cyst.