The "milf" moniker is usually used for slightly more mature ladies (30+) who are damn hot. Having children is presumed but not essential.
It has entered mainstream parlance:
en.m.wikipedia.org/wiki/MILF
Younger attractive women with children are usually known as "yummy mummy's".
There is no scientific threshold: but if you see a women with a beautiful posterior in the parking lot of the local mall bending over to strap her toddler into the car seat - chances are she is a yummy mummy. When you see a woman at the school sports match and you find her more attractive than any of the teenies - chances are she is a milf.
I would rate Arizona, Mynx and Sam as the hotest sexiest and most skilled milfs around. Mandy in Menlyn is on the border of yummy mummy and milf and amazing.