My kids are 15 and 17 now. I aggressively budgeted from the similar age as yours. What I found out.
Kids cost the most from age 7-13, then drop off a bit in costs, with a spike for graduation year (or especially when they start to drive).
1) I give kids $40 per month for allowance,since age 8, and with that, they have to contribute to some school field trip fees, supplies, and personal entertainment out of that. They also hoard it for future technology wants. Total cash budget for them was $120/month each (excluding food, accommodation, education savings, vacation and gifts, but including extra circulars, clothing, entertainment, birthday parties, camps, etc). Razors, deodorants, etc turned out to be minimal extra cost... as other costs fell off.
2) As they became over 12 years the cost for sports fell. Why? I refused to pay for rep teams, and they scaled back to just choosing 1 activity at a time, house league. One daughter chose no extra ciricular after age 14, and just did after school things.
3) After age 12, there are ways to make money -- paper route, babysitting, mowing lawns, until age 14/15 when more money opportunities (reffing, teaching an art class, painters helper) open up. Therefore, the allowance did not have to grow as they had more things to spend money on.
4) Christmas and birthday gifts -- half the time these are for a phone, or other wanted item, and they top up with their money to get the version they want. They also pay for their own contact lenses, phone plans, etc with it. No net increase, really, for tech.
5) Graduation year was expensive for my daughter -- just under $1000 including dress, yearbook, banquet, university application fees, AP test fees, graduation photos, small cake and family party, etc. That is with her paying $200+ for miscellaneous costs from her own wallet.
6) My son started to eat a lot at age 13, and eats more every day. He will not eat the low cost foods or leftovers unless I cook it up and hand him a plate. Hard to get him to make PB&J sandwiches for himself when there is a frozen pizza in the fridge for a "snack". Food costs jumped in a major way because of this. Yet he is skinny, so I am happy to have him eating, so I buy foods that work for him even if more money. My guess is $200 per month more just for 1 teenage boy.
7) Driving lessons and driver testing fees, and adding insurance onto the car at age 16... $1000 ? (YMMV).
8) Vacations start to cost more, as you want more room than a small double bed hotel room, or in my case, we are going overseas for the first time, as they are old enough to "get" it now. We also bought a larger car for the leg room for long camping car trips.
9) Ages 7-13 came with the large lumpy expenses for braces, speech therapy, or vision and other therapy issues, etc. All the stuff most parents would look to pay for kids, plan $8000 per kid for this, just in case. Sport and camp fees are the highest in this age group, before they settle down to one sport, or choose sports just through high school. They also outgrow sport gear and bicycles every other year during these ages. We also spent more for family time skiing in this age.
10) Long-term Education costs -- we saved the same amount each year from when they were babies, so no change, other than the misc. fees that hit in grade 12 for applications and college tours. (see above).
Note -- after the youngest was 2 years, we did not pay for daycare.... only preschool, extra ciriculars and summer camps for enrichment... so the comments about them being cheaper, older, is not quite true for all.