Generally, I'm not a fan of referring to one's stomach as one's tummy. It just sounds too silly. One exception to this, however: I'll admit that a chant we used with family cat Sammy wouldn't have sounded as endearing if we'd said, "Stom, stom, stomach rub." In that case, "Tum, tum, tummy rub," was more appropriate.
I can also think of one other exception.
