Foof, that's brilliant that you have got a job 
Well done!
I don't really have many tips for a cheap summer holiday because, sadly, we live in an area that has almost nothing for kids to do - no play parks, nothing. And when you have a bunch of kids and travel by bus, even things that should be cheap, aren't.
Things that go down well, here, are; garden picnics; building tents out of blankets, garden chairs and clothes pegs; playing Pooh sticks off the bridge on the walk to the nearest park; foraging for wild raspberries and woodland strawberries (don't think this will work this summer); water balloon fights; treasure hunting on walks through the wood (find an oak leaf, spot a tree creeper type thing) and making pictures from the assorted guff they collect(younger ones only).
Most of my kids are bigger, now, and want to do their own thing most of the time but when they were younger, they liked to paint on the pavement with water (no mess to annoy the neighbours), play hopscotch, elastics etc. Also a game where they had to throw beanbags into my washing basket and other receptacles placed at different distances. You can make obstacle courses from hula hoops, play tunnels, beanbags and balls.
You can make cheap ice lollies by freezing Angel Delight type stuff in lolly moulds, also the juice from tinned fruit - get your money's worth. A cheap summery snack is apple smiles - slice up an apple, spread the slices with peanut butter and top with a few mini marshmallows for teeth. Stick another peanut buttery slice on top to give a smile.
A lot of LAs have a summer programme of things for kids to do. My friend lives in an area where a lot of these are free - arts and crafts clubs, sports things etc. Also some libraries have a summer programme of events. So it's worth checking your council website.