2 kids, age 11 and 12
One does girl scouts ($25/yr plus occasional other expenses), fencing (not at a high level) averages about $150/month with an occasional tournament that might bump that up but mostly her tournaments are $50 or less. She was also taking an after school programming class which was about $75/month but taking a break from that right now.
One does rec soccer, I don't remember what this season's registration fee was, definitely less than $100. Also golf, averaging about $100/month. Equipment was expensive but like fencing it just has to be replaced as he outgrows things (and my dad's a big golfer so he likes to give him clubs, balls, etc.), Also participates in church youth group at little cost.
Both kids also play in school band so we rent their instruments but don't do any private lessons so the cost is minimal.
So, regular on-going expenses are about $250/month. Another couple hundred during the year for registration fees etc.