To answer your original question, how can you get through to him that children are expensive - you can't. He won't hear it and doesn't want to know.
Despite earning a huge amount, he has irrational fears of poverty and bankruptcy. This is not something you can solve.
Added to this, he nit-picks and criticises about your expenditure. There may be a combination of reasons for this grumpy nit-picking moaning:
Could it be he finds it an outlet for his stress, to emotionally 'dump' on you instead of handling his emotions like an adult.
Could it be he likes to be in control, and doesn't like that your spending is not in his control.
Could it be that he fundamentally disagrees with your spending patterns, that he is more comfortable living as if you were both poor, basically he is a miser.
Whatever the reasons, you can't fix him or change him.
So your choices are to divorce (ASAP for the sake of the children while they are too young to remember), or to find a way to live with it.
At present your strategy of simply not talking about money is a good one, in that it mostly works, except when you have an expensive month. So to fix that, make sure you have personal savings to fall back on in those expensive months, so that you never, ever have to ask him for money.
So cut back a bit on the spending, build up some emergency savings for yourself, and you can avoid those conversations completely.
A couple of things are very concerning:
First, why does he want you to reduce your hours at work and earn less? That doesn't make sense if he is worried about household finances. But it makes perfect sense if he just wants to control you and have you at home for his convenience.
Beware this relationship doesn't escalate into controlling abuse. Don't reduce your hours, as that will only further undermine your independence and future security.
Second, what is your long-term financial security looking like? Is your pension adequate? You mentioned that when you no longer have the cost of childcare you could put more into the mortgage: DON'T do this, you need to put the extra into savings and pension in your name only.
It is not fair for you to pay more into the household when your pension and savings are in a worse state than his. Financial fairness has to include the whole picture including the future, not just month-to-month costs. So don't pay more into the household without first having a big cards-on-the-table discussion about his financial situation and pension, and working out what is truly fair.