I think Mandelbrot was assuming that your contriution to your pension was 6%. It sounds like your employer contributes 8% from what you are saying, do you contribute also?
If you reduced to a 3 day week, the 8% percentage should stay the same, but obviously it would apply to a smaller amount as your monthly salary would be reduced.
I believe your employer will have to wait until you return from maternity leave before making you redundant. I would not entertain moving from 5 to 3 days at this point as your lump sum would then be based on the new, lower salary, so it would not make sense. If you do need to return to work before the redundancy happens, one option you could investigate would be taking an unpaid parental leave day to reduce to a four day week, while maintaining your official salary at your pre-maternity level.
If your youngest is 4 months now, presuming you would be returning to work when they are 9 months old, if you got redundancy then you would be eligible for JB for the next year (if you find something that pays for the 3 in creche then great, otherwise at least you'd have some income), you could then use the redundancy to support yourself for the year after that, by which stage your youngest would be almost 3 and your eldest (possibly both your older children depending on age) would be in school. At that point, your childcare bill should have reduced to a level that makes it lot easier to find employment that works for you.