A few questions for clarification:
Is this a riff you yourself would preform?
Is this for kids?
If it is for kids, do you realize how insanely hard it would be to do a riff appropriate to that age group? It would have to be without bad language, sexual innuendo, implied violence, or exploitation of tragedy, unless you want an army of angry parents to lynch you. Also, you'd be limited to referencing pop culture a kid would be familiar with. Riffing "Suddenly Dash crashes into XLR8. Oops!" could work if you're riffing Incredibles, but a Jacque Cousteau joke during Finding Nemo would go right over their heads. You could be opening yourself up to a world of pain, my friend.
Also, why do you think Sharkboy and Lava Girl would be hard to find?
With all that said,
Bambi- Classic, possibly well known by the audience. Lots of possible jokes as a result of it being part of pop culture.
Incredibles- A fun movie, not filled with jokes every minute, washing out the riffs.
Emperor's New Groove- Bad movie that deserves riffing. "David Spade, professional jerk."
Godzilla vs. Mechagodzilla or Son of Godzilla- They can be hard to get ahold of in places, but Godzilla films always work.
One of the Pokemon films- Need I elaborate?
But now, may I make a big suggestion?
Don't do a full length movie. MST3K broke up their movies into colorful segments because they knew adults had trouble focusing for 1.5 hours. Kids would no doubt get bored of people making jokes over a 2 hour movie.
So instead go to
http://www.archive.org/details/prelinger, find an appropriate short film, and riff that using Windows Movie Maker or it's equivalent. It's easier for you, easier for the kid, and much more entertaining to their ADD riddled minds.
In any case, good luck.