I'm lucky to be able to take a few weeks of unpaid Parental Leave each year, plus copious use of holiday clubs, some playdates.
Where I live most of the private schools offer holiday clubs, and you don't have to stick with your school - you can go to another school's holiday club no problem, state school children go there too, in fact many state parents rely on the private school clubs. Most will accept childcare vouchers which means at least 20% off cost (closed to new starters so you're probably not eligible, but there are newer tax free childcare schemes run by government which may work for holiday clubs, you'd have to check).
There are also cheaper holiday clubs, which run maybe a week at a time - coding clubs, science clubs, YMCA clubs, religious clubs...
This year things are particularly difficult - almost none of the clubs in my city are running, those that are are opening with a quarter capacity due to Covid safety so spaces go immediately/priority to key workers, they also require people to sign up for the entire holiday, to avoid bubble mixing. And costs have gone up because of the numbers/space issue.
Costwise - our local private clubs in non-Covid times are about £50-£55/day per child, run from 8-5.30 daily. Usually small discount for a whole week/sometimes 2nd child. The clubs run in state schools/church halls/science clubs tend to be shorter days (more like a school day) but cost around £30-£35.
Once the children get into late juniors you may be able to work from home if that's possible for you, whilst they play/watch TV/have friend round some days. I'd never (pre-Covid) have tried wfh with young kids before because it's a nightmare, but Covid has taught me the older one doesn't need much oversight. I'd only do this for odd days though, because they get bored. Once they're at Yr8 or so I think that while they may enjoy holiday clubs, they're probably not absolutely necessary if you can't afford it.