The percentage recommendation is only a rule of thumb that falls apart at both ends of the income spectrum (because a £500 mortgage on a £2k income is going to leave a lot less for other costs so be harder to manage, than a £3.5k mortgage on an £8.6k income, despite being a lower percentage).
I would stay - you've got a good set up that might be hugely disruptive if you move, plus it would be very expensive due to stamp duty. You're in the worst period re childcare costs, which have to reduce by something when they're at school?
You'd be best off looking at your income and expenditure and make sure you're making the best use of your money - if you aim to have the recommended 6 to 12 months in an emergency fund, obviously with your costs that's going to be a substantial amount, but worth bearing in mind - you could at least save the child maintenance if you say it's not reliable, so you don't get used to having it.
But definitely review your budget, and make building up your financial resilience a priority rather than 'lifestyle'. A couple of years of sensible budgeting should make it look a lot healthier. Play the long game, don't panic about a short term bump in the road (high childcare costs, fairly recent divorce?)
Have a look at the financial flow chart to get your thoughts in order:
ukpersonal.finance/flowchart/
Long term, you have a chance to reduce your mortgage with your pension lump sum and also your DC will become young adults and while you might end up supporting them through university, you might not, and when you can start taking your pensions, start to receive the state pension (which is another near £1k pm at today's prices) you'll have nearly or even actually paid off your mortgage and you won't need a family sized house and also not necessarily need to stay in an expensive area if you're not working/doing the school run, so then is probably the time to move to reduce costs and release capital.