If they are currently affording to rent on minimum wage jobs, it sounds like the area they live in isn't too exorbitant?
(Not being sniffly about their jobs at all, by the way - fair play to them - sounds like they are both grafters).
If she will struggle to afford the rent on her own, which sounds likely, I guess her options are:
To sub-let a room (not easy, with kids - unless she has a friend or trusted aquaintance looking for somewhere? This is assuming there is a spare room, of course).
Look for a smaller, cheaper place in the same area? Flats are often cheaper, for example.
Take on more work - but that sounds tricky given she is full time already. Is there instead a promotion she could push for?
Ask her ex to contribute a significant amount in order to keep the kids in the home they know. But this is tricky as I assume she wants him to be able to afford somewhere at least decent enough to have the kids over for however often they agree.
Will they go for a 50/50 childcare split? In my opinion that's best for the kids if he is a decent dad. But it doesn't help DD financially. But that has to come second to the kids' right to a decent relationship with both parents.
Would they consider cohabiting as a separated couple, at least for the time being?
Or doing the "nesting" thing I have heard of on here - moving out when they don't have the kids, and back in for the time they do? So she could live with you during the periods he has the kids, in the family home?
Hope they manage to sort something that works for them and the kids. I am pleased she has you in her corner - you sound like a sane, rational and loving mother/granny.