DD1 is about to become a teen, DD2 is just 9.
I work from home so don't have childcare costs so comparing like for like, DD1 is more expensive to run.
She's not the world's biggest eater so food wise, she's no more expensive
Clothes are more expensive. She's quite thin, but very tall with size 5 feet, so we're in that cross over between children's clothes/shoes and size 6 women's clothes/shoes. She's not into designer stuff, and is still quite happy with H&M and Primark with a bit of Top Shop thrown in, but her style and tastes are more expensive than her sister's. Shoes are more expensive
Days out and her social life - a lot of the stuff round here stop children's tickets at age 12. Plus she wants to go out with her mates to the cinema, etc so money for bus fares, cinema, bag of chips and an ice cream if she's at the beach with her mates. She's not quite so happy with crabbing/kite flying and a picnic like her sister still is. Equipment for her hobbies is more expensive in the larger sizes
School uniform - now she's in high school, gone are the days of supermarket skirts and polo tops with a cheap pair of shorts and trainers for PE - £47 for a blazer, £35 for a skirt, £80 for a PE kit, plus extras for things like football boots that were only worn for 6 weeks.
School trips are more expensive than they ever were in primary
She's constantly in the shower, face washes, toiletries, bits of make up, hair gubbins, even the hair dresser is more expensive.
We haven't even got into the realms of driving lessons/Uni yet 