Ah, personally I'd say it's more the age than having kids! Body types don't have anything to do with it in my opinion - but age, and I suppose a certain type of personality (at least with the MILF characters I've seen so far) make someone a MILF. Or DILF, I guess.

Children help. They imply that biology has added some extra natural hip padding, breast size, as well as implying there's lactation potential. Age also helps, over 30, even over 40 or 50, to have the kids grown and out of the house. Then the usual markers, wide hips, bit of a belly, thick thighs, generous chest. Plus, as backstory goes, having kids emphasizes her fertility, giving a natural aura of sexuality.

To me a milf is any feminine-appearing person or character who appears between the ages of 35-45 and who has thicc hips, a bosom (not necessary but tends to hit the 'motherly' buttons more) and a large butt. Think you're typical 'mother goddess' statue from ancient times. IMO they don't need to actually be able to bear children or have any. Just the appearance is fine.