£400 sounds like a lot, but to cover clothing (for a nursery aged child plus two adults) and activities, seasonal stuff like birthdays, emergencies, all parking, toiletries etc and other things it doesn't actually go that far. Do you have holidays as well? That will also effectively be coming from your £400 per month.
What's probably happening is that you're mentally going it's OK, we've got the spare £400, and you're not realising the fact that you've mentally allocated the same £100 (ish) 4x over and that's why there's nothing left at the end of the month.
It does help to get granular IME, either with an envelope method or some kind of software. If you will forget to input things, then you need something either tangible and physical (go and draw that £400 out at the start of the month, then use that cash) or digital - like an account with pots or some kind of software. I use YNAB and find it saves us more than it costs, which I didn't think could be true when I started using it, but it does. I just paid monthly for about 3-4 months and once I knew it worked for us I switched to yearly. (I did not originally link to bank - but the reconcile feature means you can cross check and keep it up to date always, even without linking).
If you will input stuff manually, you can also use a spreadsheet. There is a useful one on the MoneySavingExpert site under their Money Makeover. That gets you to break down all costs including lots of reminders for things you might not think about, and converts any weekly or yearly costs into monthly.
Look back over the last 3 months' worth - I bet you have spent more money on food than in in your main food shopping budget, I bet you have spent on things that come up 1x or 2x per year but hadn't had money put aside for.
Based on what we had set up originally I would do a rough budget/pots thing like this:
£100 "one off" costs
£50 savings
Some allocated to cover the recurring, known costs like activities - and consider cutting down if this takes up a lot of the budget
The rest divided between some pots like clothing, toiletries (that you can gain a rough average idea from the last 3 months) - and again consider cutting down if you're spending more than you'd like to here.
Hopefully - this should give you a more realistic idea how much you have to spend, and cover the things that come up infrequently, AND have a little to put aside for emergencies or savings.
With the £100 towards "one offs" this is stuff like anything that gets paid yearly rather than monthly, including birthdays, Easter, car tax, subscriptions etc, as well as less-frequent but pricier clothing purchases like shoes/coats. £100 a month for now because it's front loaded - ideally, you want to be putting money aside to cover things like this all year so that the cost evens out. But since you haven't done that previously, you'll need to keep some money aside to cover these kinds of things. IME it tends to even out at about £100-150 (though obviously this will probably depend on your income etc!)