Use your free Ebay listings every month. In the UK its 20 so 5 listings a week, about an hour a week to sort, take decent photos and write good listings.
I buy good label stuff second hand, use and as soon as its use declines sell on - often braking even/ small profit. This works for toys and clothes. It also helps keep on top of the clutter.
Teach the children financial basics from a young age. My favorite when they where tiny that DS1 (now 11) reminded me of was one icecream at McDonalds or a whole pack of 10 or bag of doughnuts from Netto across the carpark.
As soon as they can grasp number then let them have some say in a small amount of money/ week for activites and treats so they're empowered but will learn that when its gone its gone. Next week they can vary their choices.
Find out the off peak deals at activities like kids am at the cinema (very cheap showings of not the latest release films but when they're young they don't get release dates anyway). Our local playbarn is about £6 for a peak session but at a midweek toddle its £2.50 and adults get a free hot drink (many drinks are around the £2.50 mark).
Have a few bolt hole activities in reserve for when you feel stir crazy in the house. Feeding the ducks, playpark etc all fine in good weather. Free coffee at Ikea with a family card is one of my just need to get out things. Just gettiing out with no plan can cost a small fortune.
If they're badgering you for toys agree its lovely and then take photos and then put the images on a board at home. You can build up a wants list that you can see patterns on and the can save towards things on their list or you can scour sites for decent deals and buy in advance for birthdays etc.