I've been watching MST3K with my kids since they were around 5 and 7. Joel era is much better. They still have adult humor in there, but it is very oblique. Plus the Joel era just has more of that rosey saturday morning feel.
- We started with the Gameras. They are all good, but the later ones are in color and wackier.
- Both Godzillas are very good. Godzilla vs Megalon with Jet Jaguar is fantastic.
- Santa Claus Conquers the martians is also fantastic--it's actually an about par kids xmas movie but with joel and the bots (the host segments are little rough though, especially the essay segment).
- Time of the Apes is okay if you don't think the monkey makeup will scare them.
- King Dinosaur is pretty tame and there are monster fights and joey the lemur!
- The Giant Gila monster isn't too bad. Moves at a brisk pace, has songs. They do talk specifically about people dying though.
- Lassie: The painted hills is also very good, although again, there is specific talk about death--the kids dad, and then his uncle right on screen.
Oh, also, if the new season is up for consideration, my kids love Reptilicus and Yongary, although both are probably a bit more mature than if I was still picking for them at 5 and 7.
My son is now 12, and this summer we started into the spy movies and some of the other black and whites that he likes just fine.
Hope that helps. I LOVE watching MST3Ks with my kids, and my son especially.