I have one kid so I have a little bit of perspective on this.
Looking back on things, I don't think I've lost too much time by having a child. It's a little inconvenient sometimes having to shuttle him around. But from my observations, single/childless people tend to spend the same amount of time on extracurriculars that I spend with my child.
Really? How active a part do you play in your child's life? Because much of my non-work time is spent caring for my children. Even when they're asleep or not in the house, I am doing things for them or because of them.
I have 2 children and found the workload goes up significantly on the second. No economies of scale there, in fact the reverse as with one you can take turns with your partner to look after them but with 2 it's typically all hands on deck.
Also regarding extra curricular, people might spend the same time socialising etc but this counts as time to re-charge and get back to work. If you spend a couple hours getting your kids dinner and into bed you don't feel like heading back to the desk the same way you do after a game of tennis with friends.
Looking back on things, I don't think I've lost too much time by having a child. It's a little inconvenient sometimes having to shuttle him around. But from my observations, single/childless people tend to spend the same amount of time on extracurriculars that I spend with my child.