As per po, you really need to do a full financial audit to understand where your money is going, unless you do a lot of your day to day spending using cash this shouldn't be too difficult, just time consuming. Set yourself up a simple excel sheet with categories like 'takeaways', 'top up shops', 'kids activities' etc and download your last 3 months bank and credit card statements and go through line by line and assign it all to a category.
Once you have a good understanding of where the money's going, you can make yourself a monthly budget. You need to include in this items which are actually less frequent than monthly expenses like kids birthdays, holidays, Christmas, car servicing and MOT. A bank account that has a 'pots' function like Monzo is really really useful for this, I have about 10 different pots set up and as soon as I'm paid every month I transfer money into them that is 1/12th of the annual amount I want to spend, so when e.g. Christmas rolls around my money is there ready to spend (this also helps me not go mad and overspend, if you're putting something like that on a credit card to be paid off later the temptation can be to say fuck it and worry about the cost later whereas if you've saved all year you want to spend wisely). After my bills and essentials are out and my money to my annual pots is gone, plus a fixed % that goes to longer term/rainy day savings) what's left is for monthly disposable income. You might need/want to further sub divide this using a spreadsheet you monitor on a weekly basis or again a 'pots' function, or an app like 'you need a budget' into categories like takeaways, clothes, subscriptions, kids activities. Whatever method you use the crucial things is to understand what outgoings you have and keeping to within your income, so e.g. if you have £500 a month to spend but have £450 committed outgoings on kids clubs, gym membership, sky TV, phone contracts etc that's fine, but really only leaves enough for one takeaway and a couple of small top up shops a month, so it's no good going out for a big blow out meal on payday and then clothes shopping, then wonder why you've got nothing left by the end of the month. So if that's the issue you have, you need to have some realistic conversations as a family about what's important, can some of the subscriptions or activities go, or if not then that's fine but there's then no slack in the budget for other treats as well (or somehow you need to increase your income as a family is the other alternative). It's not about forbidding things or living a Spartan lifestyle but about prioritising what's really important and not wasting money on things that actually aren't like memberships or subscriptions you don't use or clothes you don't wear or food that doesn't get eaten...