I hope you terminated their m2m in a timely manner as you only did so early March, rather than late Feb. In any case, you need to end your "relationship" with these tenants - obviously. While I understand you not wanting to put her on the street, the likelihood of the bf getting access to the place once he's out is 99.9% if she stays. She will not (be able to) refuse him access, especially with the kids also being present. So you'll end up with him on the premises again, and then all hell breaks loose, one way or the other, considering his background. She has to leave.
I would therefore never extend her right to stay, irrespective for how long and in which manner. No one has to live on the streets, there would be a shelter if push comes to shove. Then there will be friends, family or whatever other way of getting a roof over her and the kids' heads (and no, not one of yours ;-) ). Tough situation, yes, but she also got herself into this somehow.
If you want to assist her with selling the car, then that might be a nice gesture and good support (possibly), benefitting both sides. But I would draft up a contract with her for that, a simple one, making it clear that she agrees to this and is not being coerced by you to do so blablabla. Kindness more often than not backfires in such situations if one is not doing it properly and protecting oneself.
Good luck.
Oh, and also, as they clearly caused issues and especially since they are m2m: why the heck did you keep them???