Apart from childcare costs I would say once they reach year 6 in school and have their first residential school trip.
After that it's high school uniform (no more polo shirts and skirts/trousers from Asda), shoes cost a fortune and they grow out of them rapidly. All the clothing costs escalate as they know what they will/won't wear and you can no longer get hand-me-downs from friends etc. They grow, they eat endlessly, their school trips become very expensive.
I do laugh when people say that they won't spend £'s on their teens. You will. You will want them to have the opportunities to do things like DofE, the NCS scheme, the activities that keep them busy and off street corners.
DD1 is 19 and living at uni. She still costs me money. I help her with fuel for her car when she's short, I help her with buying books and I often take her food shopping when she's home so she has stuff to take back.
DD2 is 12. Two residential trips this year, one in 3 weeks time to France, the other in July. Total cost of £700. She requires a wetsuit and other stuff for the second one which we already have although she needs a new rash vest and wet shoes. Next year is the ski trip which she isn't going on, she got the choice of going on 2 out of the 3.
She doesn't start high school until year 8.