For things to do for the kids, have you thought about signing up towww.showfilmfirst.com/create_account.php? It does depend on where you live (bigger cities get more offers) but even here in deepest darkest Somerset we get offers for tickets to cinemas and occasionally the theatre.
Have a look atwww.moneysavingexpert.com/family/school-holiday-deals/ - and on the vouchers, yes, they're laborious and if you work out the hourly rate sometimes it's ludicrous but you can do them while you're watching TV or waiting for the bus or whatever, and several of them offer PayPal and other vouchers as well as Amazon. A couple I do semi-regularly are Qmee and PrizeRebel, others include Swagbucks and ipsos i-say. More tips at www.moneysavingexpert.com/family/make-money-surveys/
Have a look atforums.moneysavingexpert.com/discussion/5606097/free-cinema-tickets-codes-only-no-chat as well.
And on the food thing, we pretty much always check mysupermarket.co.uk to see who's got offers etc, always make use of Iceland multi-buys and Co-op multibuys, check out end of lines in B&M and Pound stores and so on; I have also looked at Approved Food in the past but found it was better for "treat" food than everyday food so we ended up not using it. I very much miss living in a town with a proper undercover market like where I grew up where you could get bags of fruit or veg for £1, though some of the supermarkets do this now.
And I wouldn't be without our slow cooker and blender, I just wish we had more freezer space! Also, I've learned over the years to bulk out meals with soya mince, grated potato, breadcrumbs, grated veg, barley, noodles, pasta etc.
skintdad.co.uk/deals/ has some good ideas as well and for petrol, look at www.petrolprices.com/ for the cheapest petrol near you. You'll need to register but it's free and they don't send you marketing stuff every five minutes or anything.
Another fun activity that doesn't cost much (though it's not free) is to get a bus or local transport day pass and spend the day bus-hopping in your town/see how far you can travel in your county etc.
And as a single parent, there might also be some useful links relating to holidays atwww.familyholidayassociation.org.uk/apply-for-a-break/other-holiday-charities/
And have a look at your local libraries and museums and councils and universities to see if they're running any activities that might help you all get out and about. It sounds like you're doing so well, best wishes to all of you.