There is an important age distinction between a yummy mummy and a MILF. Yummy mummys are younger than 30, while MILFs are older than 30.
Also note that the plural appears to be 'yummy mummys', and not yummy mummies, as is the case with 'mummy'. Additionally, it is interesting to observe that that it is 'mummy'and not 'mommy', a distinction that is often lost on this Indian-flavoured brain.
And finally, this word doesn't seem to be hypenated.