I actually liked the book Savage Sam better even than Old Yeller. But the movie was poor and here are three key reasons why:
1) Dorothy McGuire, Fess Parker, and Chuck Conners were missing, even though their characters (Katie & Jim Coates, and Burn Sanderson, respectively) were key in the book. They were replaced by an uncle who came out of nowhere, played by Brian Keith. It really damaged the continuity of the two films.
2) Secondly, in the book Savage Sam was the son of Old Yeller, but in the movies this is obviously not the case. The puppy at the end of Old Yeller is clearly not a hound dog, and Sam is clearly not the offspring of a Lab mix like Old Yeller. The movie explanation of what happened to "Young Yeller" as well as where Sam came from? None is offered.
3) Finally, and most important, too much time had passed between the making of the films. Tommy Kirk and Kevin Corcoran were clearly too old to be playing the young Coates brothers by the time Savage Sam was made.
Pity, because it could have been a wonderful film.