But the OP isn't about a child that wants to do extra sport. The OP is about a child who doesn't want to do more than the tennis she already does.
Yes, it's great if your child wants to do it. Bloody shit if they don't enjoy it. What I was getting at is that the middle ground is being lost.
The idea that kids need to be doing organised sports or they'll be slobbing in front of the TV all day. Organised sports can feel , to some children, not all like yet another place where they have to be, have to do something to a standard, get chivvied about. For 5 days a week, at school, they are told where to sit, how to behave, judged on their work. Organised sport can be an extension of that in the eyes of a child who isn't that interested. You can be bored doing as much as not doing if you aren't into it.
Kids also get bored easily if they never learn how to entertain themselves. You can bet the bored kids you are talking about weren't allowed out much on their own, weren't permitted to spend much time without their parents, etc.
Kids are naturally good at playing. It's what they do. Put any group of kids in a big field with no tech and watch. Not many whine they are 'bored'.
I used to take kids to forest schools. Kids who had PlayStation's, the works at home. Kids you'd think would be bored to death. You know what they loved? The time we gave them between the learning projects, to play. Nobody was bored. They used their natural environment to create their own sport.
We don't give kids credit or space to be kids. We don't let them be bored, so they don't learn to alleviate it. Part of the problem is the stuff they have, now, but you can take them out of that on a Saturday afternoon without needing to sign them up for two hours of netball.
Kids need this, now more than ever. The chance to just BE kids. To get breathless playing tag with no rules other than what they make up.
Kids are very easily entertained if we let them find out for themselves, instead of telling them how to do it all the time.
Our kids are becoming passive vessels, waiting for adults to fill their time, tell them how to use their energy. I'm not saying there's no place for organised sports. There obviously is. I'm just saying if your kid isn't interested, it might be that they are craving a freedom you just don't get in organised sport. The freedom to play.