alark no, I am not saying that all SAHMs of school age kids are U. If a family has decided that they can afford to do this, it is up to them.
In this case, the family has decided that the strain on finances isn't sustainable. I don't really see the evidence that the DH hasn't thought it through, I know the OP mentioned cars and holidays but she was I thought saying that generally things are just too tight. Gas and electricity rises, petrol rises, kids needing stuff at school- there are all kinds of things that could lie behind that decision. The OP hasn't said she disagrees with the basic decision that the family needs more income, she just wants to find the right way forward.
Which I agree with. And it's what I did myself. But it took a while to get back, and it is true that if I had stayed in my original career, we would have been rich
. But I gave up work for seven years, worked PT for five years (25 hours IIRC) and full time now in a new career, earning not what I might have, but enough.
My advice is to hang onto that core job like grim death because it is key until you find the way forward, but if there is pressure in the meantime, to supplement the hours.
As for getting into student support, it depends so much on how it is run by the Uni the OP works for. But there may well be a student support element of the current library role, and that's where I would go. IMHO the great thing is to have some specialist knowledge. Texts, and how you get ecopies off publishers, and accessible reading devices, are all key issues and she is well placed to do that.