Going on your first post, with the things you mention (2 lots of music lessons, ballet lessons, riding lessons, school trips, constant birthday parties, shoes and more shoes, clothes), here's how it could change:
They stick at the instruments, and need longer lessons. You wince but agree to stretch to this, because they're doing well and are now really enjoying it.
They take music exams. These cost more as you go up the grades, and you may need to pay the accompanist, or for extra practice.
The teacher says you need to upgrade the instrument (tip: start them on the recorder or triangle. Do not go near the harp, tuba or bassoon.)
You ignore the advice above, and need a bigger car to transport the tuba, harp, double bass etc.
They join an orchestra, and it becomes their main passion. They come home one day wildly excited because the orchestra is touring abroad. In velvet dinner jackets. Oh, and they are the only tuba/harp/bassoon player there, so Mrs Barksworthy is going to be very disappointed if they don't go.
Dance and riding lessons: as above, substituting 'horse' or 'tutu' where appropriate.
School trips: you refuse to consider the trip to Hawaii, the Alps or Russia. Feeling mildly guilty, you say yes to the camping trip to Snowdon, then find they need a tent, boots, sleeping bag, midge nets, more boots because they've already grown out of the others...
Good luck!