OP, I love your attitude - looking for things to make life better instead of feeling bitter. You sound lovely and really impressive.
Don't be shy of looking on Freecycle for everything, even Christmas presents for kids. We have given away really gorgeous toys the DC have outgrown, and have been given as-new musical instruments, skateboards etc which would make fantastic presents, along with watched once DVDs etc. Just ask.
Always put a % of money aside each week, however tiny, for treats. Even if it's just a pound a week, spend it on sweeties and eat them with the kids or buy a bag of popping corn and a charity shop DVD to have a film night. Always make it a % not a set amount, so if you suddenly have an increase in money, you get a better treat, and if you're broke, you don't stop it altogether.
Make a list of things you love to do as a family (or for yourself) that are free or almost free and do them often. Take kids to choose library and audio books once a week and read them together, go to the park, go blackberrying and make a crumble or pie, go cycling, go for a nature walk, stare and clouds or stars, build snowmen etc. Don't ever feel you are providing less of a childhood for them because you can't afford things. The best stuff is usually free, we just get too tired or down to remember to do it.
When the kids are teens encourage them to get part time jobs to pay their way.
And look for ways to earn more and to get richer, don't ever feel you are trapped in poverty because your marriage didn't work out. Could be the making of you.