Something that tastes good.
Donuts are very nummy, as are potato chips.

See also: Yummy, Delicious

Nummy requires less lip/tongue/teeth coordination and movement than Yummy, and so kids are generally able to make that sound much sooner than yummy.

It can be used and changed as needed. When feed an infant, for instance, one might (as the spoon nears the mouth) say, "Mmmm! Yummy! Num num num num num..." until the child opens his/her mouth and eats the food.