Those ages there are so many free things you can do.
Parks, beach, walks, library, museums, see if you have any free fun days locally (fire station, parks, church halls etc).
Take drinks, lunches and snacks on days out so its just entry and travel costs. Avoid gift shops.
Do you get universal credit/tax credits? There's quite a few offers if you claim them. (Kew gardens or wakehurst £1 entry, whipsnade/London zoo £3 entry, London transport museum £1 annual pass)
Eating out, see if you can find any of the kids eat free offers.
If soft play is a regular place you visit is it worth doing an annual pass? Do you have discounted or free swimming places near you? My city is free swimming for under 18s at certain pools. Or is it worth getting a membership for the pool so you could go more often?
Utilise discount codes - days out vouchers website if you use the train (2 for 1 entry). Clubcard vouchers. What jobs do you and your husband do? There might be discounts for certain professions. I saw a boundless card that offered great discounts to key workers.