Sounds like a really terrible idea for her, and the kids. You say that Dad threatened to kill her AND THE KIDS; the kids didn't "cheat on him" or do anything else to him.
A better idea would be for her to leave the restraining order in place, and with it still in place, to work out a COURT-ORDERED schedule for Dad to have time with the kids. IF Dad can get his act together, AND Mom can get her act together in terms of dealing with Dad (which means understanding that there's some middle ground between sending him to jail, and living with him without any rules) then it may be possible to fix things, IF everyone starts acting consistently like mature reasonable adults.
Mom can dismiss the restraining order if she wants to.
That means that if Dad blows up again, a court will be much less likely to grant a new restraining order.
It also means that if Dad blows up and hurts the kids, or threatens to hurt the kids again, there's a risk that the Court could find also that MOM failed to protect the kids (she had a restraining order protecting the kids, and dropped it) and the kids could end up in foster care.
If Mom were my paying client, where there are kids involved, I'd insist that Mom go to court and explain to the judge why she was dropping the restraining order protecting the kids, before she did so.
Mom should talk to an attorney, and maybe a counselor or therapist, before she does anything to change the terms of the restraining order.