Welp, this is mumsnet where people regularly appear to be wail that their six figure salary doesn't go far enough so this should probably have been anticipated but for what it's worth, OP, I'm with you.
For those hard of thinking:
If you lose your job in pregnancy or with a tiny baby (shouldn't happen but still does sometimes - companies close down) then the absolute earliest nursery funding would kick in is two, but you might well be forced to work before that time. And if you're heading bavk with significant caring responsibilities the chances are you're going to be in low paid work so might well be making less than you pay in childcare.
"Working around each others hours" is only a possibility if the jobs exist, take little account of things like travel time, and don't take any account of differing salaries. If the only job available to work around your partners hours is a minimum wage evening job then does it make more sense to take it or for your partner to use that time to take higher paid overtime?
SEN is not instantly diagnosable and not all issues that take a child out of school are SEN. My son had serious eye issues for a couple of years which included regular assessments followed by an op after which he was off for a few weeks for recovery. It wasn't by any means SEN but meant me regularly taking a full day to drive to the (not local) hospital for assessment until they judged we were good to go - and then I needed childcare for his recovery. An understanding employer would give me the time but not all employers are understanding.
Schools do not instantly go to SEN diagnosis when there us SEN and a lot of schools with excellent OFSTED ratings are really crap with SEN. A friend's child who was eventually diagnosed with autism spent two year with his mum being called in almost daily to help him after accidents because they couldn't or wouldn't help. He's now at a special school - but it took an awfully long time to find him a place at one, several years before there was a diagnosis that allowed her to claim it as a disability, and good luck finding an employer that will let you dash off daily for this.