Thanks for all the replies.
Cushion not removable.
The leather is fine - just the material underneath is torn.
The damage happened with an excited 13 year old bouncing up and down playing Fifa on PS4.
Since posting, I did a search on youtube. I saw an American video selling curved needles. It 'seemed' pretty easy.
I've ordered curved needles as part of a kit on Amazon. So will give it a go in a day or 2.
@coffeeaddict that looks like torn leather. Whatever the material is it's stabilizing the seams. I'd suggest replacing it with something sturdy in a similar color and use some suitable glue.
I hope you ordered curved
leather needles for hand sewing. John James does a nice one that is just bent that would be excellent for this. Curved upholstery needles are very sharp and pointy, not ideal for using existing holes. Two types of leather needles, one with a triangular tip that cuts through leather, piercing it's own hole. Not what's wanted here. The others have a rounded tip that's tapered but not sharp. Best choice for your project.
Thread. You'll be wanting, ideally,
nylon upholstery thread. Next best is 100% polyester upholstery thread. After that, top stitching thread. Whichever, best color match to existing thread. Don't use a regular sewing thread, especially if you don't stabilize the seam. It greatly increases the likelihood of it popping out again. Or ripping along the stitching. Especially if the kiddo does a repeat performance.
As
@BrendaP said overlap the existing stitches. I'd do more than she suggested, maybe 10, or 2 inches, which ever is greater, as this is a load bearing seam.
Subtract the repair costs from the kiddos allowance.
Oh. Get the kid one of those little rocker gaming chairs. Keep him/her/they off the leather sofa. They can jump up and down on the floor. Like the rest of us.
Good luck.