Do not get legal advice.
Send him emails showing that you don't really understand. Ask in the emails how it protects you and the children.
His answers will shaft himself good and proper.
Sign it. Just make sure that you don't really understand what you are signing, on record. It would help to point out that he has made that money with your support, and that you have been SAHM for nearly all of the 13 years.
He would be F&ed in court, and his 'pre-nup' would mean nothing. If you don't split up, then its just a piece of paper.
In the mean time, live a good life.
Are your finances equal? Does he pay the household bills from a join account? If I were you, squirrel away what you earn in your part time job and learn about money and investments.
There was a truly depressing presentation in woman's hour about how having children make women financially vulnerable pensions and old age wise when the
'you support me in my career and I will protect you' understanding breaks down because women have used their energy looking after children, and it has nothing to do with educational attainment.