There was a hip-hop song a few years back, the chorus of which was a request to “let me see your tootsie roll.” The accompanying dance involved dancers – female, all – rocking and grinding their pelvises in a provocative fashion. As best as I could understand, “tootsie” in that case did, in fact, refer to the posterior.
Now, whether or not this was what made this movie scene so funny to the youths in question, I could not say.
According to urbandictionary.com, “tootsie” (and tootsie roll" and “tootsie pop”) is associated with a number of feces-related concepts, but not the backside per se. This alone is enough to start some giggles. There also “Tootie” (or “tootie fish”) for a woman’s (or girl’s) ummm, frontside.