Great advice from @Saffy101 (and some others) on this thread already
After 13 years together, unmarried and working part time with three children, you are in an exceptionally vulnerable position.
Marriage would be an excellent way forward.
The advice to stall til it's too late is not sound
Start by informing yourself of the law , from reliable sources. Check that he already intends to include wills, POA and arrangements for the DC should you both die before they are adults. If not , make sure these are all included.
He will have arranged a draft pre-nup. Ask for a copy and say you need time to read it. No one could object to this. If you are put under any pressure to sign anyway you would have the defence of duress and it would not be valid.
Getting married gives you significant protections. The time to refuse to marry him if he insists on a pre nup is long past.
Do not cut off your nose to spite your face by refusing to marry him if he insists on a pre-nup. It would be to your detriment, in the event that you did split up, to not be married.
The chances of the pre-nup reducing your share in the event of a divorce below what you would already Are entitled to as an unmarried partner is frankly zero, so it is hard to see how it could be to your detriment to marry with a pre nup.
The exemption from inheritance tax that comes with being married is also not to be sneezed at.
Please, start by getting yourself informed.