It varies depending on age.
The first set up can be expensive, even with hand-me-downs we spent around £1000 the first year for things like pram, car seat, bed. That was 16 years ago.
Childcare is the next big thing. It varies where you are and how you need it.
Primary school I actually found more affordable, uniform is normally cheaper. unless you. have to get everything from the school, not a lot of supplies are required, school trips are cheap until they get older.
Secondary school - uniform costs more, they need their own supplies and books, access to a computer during the afternoon, evening and weekend. A phone. Trips are more expensive.
Hobbies - that's where it's up to you. Swimming was non-negotiable as school teaching is basically non existing. Dance is very expensive with outfits and exams, Scouting (from 4 onwards) is a fairly low-cost one. You could try and look up groups in your area to get an idea.
Holidays and trips - again, it depends. We like to travel but we can afford it.But the older they get the more expensive children are. We are now paying 2 rooms and before that it was a suite or an appartment with two bedrooms. And you pay definitely more in school holidays.
Days out are expensive. We often do 1-2 big things per year. We had Merlin passes on year as a family Christmas present and so just had to pay for food we took with us and a coffee or ice cream there.
My PIL love theatre and concerts and often will give us tickets for birthdays and Christmas.
Clothing - the bigget problem is the jump from children size and no VAT to adult. DD started wearing adult clothes when she was 13.
Friends hadn't counted that they need a car upgrade when they had an unplanned third baby as they couldn't get three car seats into the back.