While riding her bike my daughter's pedal came off. It appears to be stripped. This has happened to her with a total of three bikes. What could be causing this to happen?
The only time I've seen pedals in a newish bike strip out of the crank was when the mechanic who assembled the bike overtightens the pedals (the mechanic was a younger me, but you live and you learn.) Often, people will forget that left pedals are left-hand threaded and will strip them out when they try to remove them.
The mechanical motion of pedaling causes metal-metal mating surfaces to fret - microscopic motion back and forth causing noticeable wear. If this were a bike from the 70's with aluminum cranks it wouldn't be a surprising failure mechanism. However, since these are new bikes, the most likely explanation is that who ever assembled these bikes tightened the pedals too much. There is no way a child could be riding the bicycle enough to cause a pedal to strip out. To have it happen three times is incredibly unlikely, though.
I'd avoid buying department store bikes and refer to your local bike shop for any repairs or replacements.