I'm taking notes since my oldest is 12 and my next is almost 11.
So far we haven't seen any outsized kid expenses. The biggest one we did see was the middle school $150 overnight field trip fee to travel 4 hours to a residential camp by the ocean. That's all the field trips for the year for her though. In the past we were spending $20-25/yr per kid on field trips but that was elementary and a lower income school. My money-conscious kid's thoughts on the $150 field trip: "it was fun, really fun but probably not worth $150 since many of my friends didn't go. I liked eating the food."
6th grade for the oldest involved an extremely intense core math course that covered 6, 7, and most of 8th grade curricula so we kept after school activities to a minimum, therefore zero sports or club-related fees (other than the occasional swimming or roller skating admission fees).
We went in for a braces consult and the quote was $4800 I think. So far we're opting to wait because the teeth might grow into place straight enough and I'm not sure my daughter is willing to take care of the increased dental hygiene and post-braces retainer usage (so might be 4800 mostly wasted dollars). The situation isn't that visibly noticeable as is, but we will definitely go for the braces if it makes sense.
Car insurance and an inexpensive reliable, safe, used car is the other big expense. We will probably get one car for the two older kids to share since they'll be at the same high school together. We're a 1 car family now so going to 2 cars won't make our overall transportation expenses go crazy. City bus / uber / bike is also an option
We do extensive family vacations. Usually a long summer trip of 3-9 weeks and 1-3 shorter vacations during the school year, usually 5 to 7 day cruises to the Caribbean. Kid-related costs are big but budgeted for. For the long vacations we do airbnb and find 2 BR places that are reasonably priced. YMMV on other kid costs. In Europe this summer, kids often travel free/cheap up till age 12-15 or so, and many places offer discounted rates for young adults up through age 19-26 or so. I've also seen many "family" ticket packages that work out much cheaper than the 2 adults / 3 kids tickets we are used to buying. I'm not sure how long the kids will want to join us on these big long trips, so our kid-related travel costs might actually drop down in several more years if one or more kids stay home without us (summer job, camp, volunteering, educational stuff, etc).