To not return their balls?

[Shadowboy]

We have an Upside down L shaped garden, and at the bottom to the left of the ‘L’ is our glass green house. It’s a beautiful Victorian style one. About a month ago the neighbours lad managed to smash one of the glass panels when he kicked a ball into the garden. We saw the lad do it and asked him to tell his dad to pop round to collect the ball and discuss the broken glass. The green house is not near their garden - it was a hefty kick over!

No one collected the ball. We paid £20 to replace the glass and carried on as normal. Yesterday it happened again - this time we didn’t see it but the ball is in the greenhouse. I’m not giving the balls back ( I’ll let the dog torture them) if their child can’t be careful and keeps smashing our greenhouse then their loss.

I’d like to add that we have kids and in the 5 years we’ve had the kids they have never damaged the greenhouse - partly because it’s fenced behind a 4ft picket fence.
AIBU to keep any balls that come over our 6ft dividing fence whether they break anything or not until they learn to be more careful?

Timide it’s not ‘revenge’. I’m just going to leave it in the garden which means the dog is likely to play with it- it rained yesterday so no one has been out in the garden so it’s still in one piece on the lawn.

The point I was making is if the kid can’t look after his ball, then why should I have to return it, particularly if he breaks my greenhouse with it. Tomorrow when my OH is home to watch my children I’ll go round and see if the parents are home and explain.