This has been covered extensively in an earlier thread a few months ago but I have banged a few single mums for a few weeks at a time, never anything serious though.
I think you just have to remember that her kids are her No.1 priority (well I hope so).
So while you're sniffing around trying to pound that puzzy, really, you are No.2 and always will be.
If you can handle that, go for it but don't be pi$$ed off when she keeps ditching you because she can't find a sitter or because she has to get up early to take the kids to soccer practice.
EDIT: Lol, I basically just repeated Danger's advice but I didn't see it before I posted. Good sh!t, Danger.