Don’t beat yourself up OP, the kids have already done more than enough. We spent years doing bugger all in the holidays due to a lack of money.
Could you say to the kids, let’s find some fun things to do that don’t cost anything, then any money I have left from the £80 at the end of the holidays I will put into the summer holidays spending pot so we can do something nicer then? That way you can add that to whatever you have available for the summer and can break up the longer holidays a bit!
For free/cheap things you can do this holiday:
”The Chore Games” - write all the different household chores onto individual pieces of paper with a point score between 1 and 3 based on how quick it is and/or how hard it is. Fold them all up and put them in a bowl. You tell the kids they get to draw 1 chore at a time, and once they complete that chore, they earn the points on the ticket. Most points at the end of the set time, or once all complete, wins a prize (you set this - I used to do a £5 voucher for their games console, but you could do an ice cream, extra screen time, money to spend on a new book or toy, favourite sweets/chocolate, etc. I was surprised by how much my kids loved playing that - and a bonus it got my house cleaned for the low, low income price of five quid 😂
Visit all the parks in a certain area - we used to go for walks around our town and aim to visit every different park, play there, then go to the next. Lots of exercise, a bit of variety and it’s free!
Go for a walk - possibly play Pokemon Go whilst walking if your kids would enjoy that?
Museum visit - most museums are free
Do you have a lake/canal/river you could walk to and feed any ducks there?
Home cinema afternoon - choose a film to watch (or do it daily and each child picks the film on their day), make the room as dark as possible, set up comfy seating with cushions and blankets plus have snacks and drinks at the ready. Sit back and enjoy the film.
Karaoke - you don’t need a karaoke machine but if you do, great! Pull up Spotify on a device and have the lyrics page on. Use them to sing along to favourite songs. if you don’t have Spotify, look up music lyric videos on YouTube.
Home disco - turn the lights down, the music up and have a good old boogie
Story time - sit and read a story to the kids then talk about it afterwards. If they’re older, get them to spend some time reading and then all talk after about your book, did you enjoy it, etc.
Lego building competition (or Mega blocks, sticklebricks, play doh… anything you you can use to create something)
Board games - old family favourites or something new? Visit a charity shop to see if they have any you can pick up cheaply if you want to try something new
Go for a bike ride
Play in the garden if you have one, or just some time to play indoors with their own toys.
Loads of things you can do that don’t need to cost the earth. If you go out, take a picnic with you or make sure you’re close to home at meal time. What really matters to the kids is spending time with you - give them your attention and they’ll be happy!