We bring home about 6k in total between us. 1k mortgage, 1.5k childcare fees, 1.5k other essential bills, petrol, food. So 2k left for discretionary spending/saving
Do things like insurance, car insurance, servicing, MOT, repairs, clothes for DC etc, ie anything that is essential but not a regular bill, come out of the £1.5k or the £2k left over? If the latter, that's money out of the £2k already, it's not all free money.
Also, as you've already identified, things like holidays and Christmas which, while non essential, need to be budgeted for, before money is considered to be available for 'frittering'. Something that's good to bear in mind is that a few quid a day on things like coffees and lunches can add up to the cost of a holiday if it's done every working day, eg a tenner a day is £200 pm or over £2k per year, that's the cost of a holiday.
Then you have to consider the cost of things like car replacement and home improvements. While there may be an element in discretion in these costs, ie you can manage with an older car or a leaky roof/drafty windows, there comes a point when these things could need a significant amount of money spending on them, so if you don't have savings, a big loan could be needed. So hopefully none of this is needed in the next few years while you're coping with maternity leave and peak childcare costs.
But I think I'd be thinking about saving £500 pm for an emergency fund/annual costs/holidays etc, and £1000 pm to cover the cost of maternity leave/get a head start on the time when you're going to be paying for 2 in nursery, and keep back a maximum of £500 pm for general day to day personal and family spending on non essentials.
TBH, I'd worry less about overpaying the mortgage in the short term. If you save most of your spare money 'just because' and use it for maternity/childcare costs while needed, you'll mitigate some/all of the interest charged (what's your mortgage rate?) while keeping the money available for the next few years, which are likely to be tough financially due to peak childcare costs.
Then when both DC are at school and you're only paying for wraparound care, then you might be able to start overpaying the mortgage.
But if your DH is wanting to spend more than a smaller proportion of your free £2k, which might be nothing of the sort anyway, he's a fool.