Hmm. Around here (middle/south USA), when I hear "tummy," I think of a child or close family member talking about their stomach organ ("My tummy hurts"). An adult would generally say "stomach" instead of "tummy." When I hear "belly," I think of the whole abdomen, including the flesh/fat on the torso. Belly is often used to describe someone's abdomen when it's able to jiggle ("What a cute little belly," probably regarding a small child/infant), protrude out far from the rest of the body (a pregnant belly or a "beer belly"), overweight bellies that stick out or hang ("He needs to go to the gym and work on that belly!"), or other variations. I hope this helps.