The answer to why the kid had to fight the giant is actually answered in the Jan 18 strip. In Greek mythology Periphetes robbed and killed travelers on the road between Athens to Troezen. In one of Theseus adventures he battles Periphetes. Oop is inadvertently ensuring history (or mythology) happens as written or at least close to how it was written.
I always enjoy that aspect of the time travel stories of Hamlin and Graue. Unlike Bender defender Firebrand, I do not see the time travel aspect that used to be part of Alley Oop as a crutch.