Yeah American porn pretty much made it that any woman 30+ with or without kids could be a MILF but there isn't really any strict criteria.
How do you know the kids aren't off-screen? It would be a bit weird to be stood there whilst the mother is taking a hammering.
In response to the question, I take it literally. A mother
I'd like to fuck, whether she's 40 or 18. If she has a kid and is hot (and of legal age), she's a MILF.