Personally I think Belly is more correct that Tummy. but I remember my mother using tummy rather than belly. ''Dear have you to got a tummy ache''? Though tummy is a more child-like speak than belly when speaking to a child. .
Etymology:
From Middle English bely, beli, bali, below, belew, balyw, from Old English belg, bælg, bæliġ (“bag, pouch, bulge”), from Proto-Germanic *balgiz (“skin, hide, bellows, bag”), from Proto-Indo-European *bʰelǵʰ- (“to swell, blow up”). Cognate with Dutch balg, German Balg.
Noun:
belly (plural bellies)
The abdomen, especially a fat one.
The stomach.
The womb.
The lower fuselage of an airplane.
The part of anything which resembles the human belly in protuberance or in cavity; the innermost part.
The belly of a flask, muscle, violin, sail, or ship
The hollow part of a curved or bent timber, the convex part of which is the back.
The etymology of Tummy - Imitating a child's attempt to say stomach, via archaic colloquialism i.e. stummy.