There are several possibilities.
If a baby was in the birth canal too long or if the mother pushed too hard she could have sustained nerve damage.
Alternatively, the pressure of pushing during the delivery may have caused some brain issue (for example, a stroke) which in turn resulted in the paralysis.
See if you can get her up and about. The sooner she gets going again the better your chances of recovery.
If you let her loaf it will become permanent for sure.
It may already be a permanent condition through no fault of yours or hers; giving birth is a dicey proposition for all beings, human, rat, whatever.
If it is permanent you should keep her alive to nurse the babies but once they are weaned, you have to decide if you are going to put her down or let her live it out like that.
This is something that only you can decide but if you’re seeking advice, here’s mine.
If she seems OK with her life as a paraplegic and you are OK with the additional care she would require then it’s fine to hang in there with her but truly understand that this will be a lifelong commitment to higher level care for her.
If she seems miserable or in pain or you don’t think you will be able to care for her, then you know what to do. This will be difficult for your son of course but part of being a good pet owner is knowing when to let go and putting the pet’s needs first.
This will be a tough life lesson for sure but some of the best lessons are the hardest.
A vet will be able to help you access this situation.
Good luck, hope it turns out well.