It really depends.
Childcare are fixed costs you have every months. Teens are expensive but not as regular and you can save up for this.
They will grow out of children clothes sizes, DD was 13 when 15-16 at M&S didn't work anymore, so you have to pay VAT. Her friend was in shoe size 7 by 13.
Less second hand options unless they are going Vinted all the way.
Eating out - most kids meals won't feed a 11 year old.
Holidays - often entry prices are adult ones. They won't be sleeping on pull-out sofas so you need a second bedroom and that could mean no free child place. Activities are more expensive.
Hobbies - the sky is your limit. DD loves Scouts and I agree it's fairly cheap. Others you may start getting used to throwing money at start very young like riding, dance, performing arts.
School - that's where the money goes, more expensive uniform, bags, equipment has to be bought, books maybe.
School trips can be eyewatering, not everyone goes but on the other hand it can be a fantastic opportunity.
You will have to save for a laptop and phone.
Primary school was one of the cheaper years despite wrap around childcare, cheap uniform, hardly any school costs, happy with cheap days out, holidays were easy.