Tell them- 'Excuse me, I'm sorry to bother you, but can you ask your children to stop throwing stones into my pond, please?'
Then if they go,'Meh' and don't do anything, repeat the question. If they STILL don't, then you have licence to shout at the children. Shout at the top of your voice, 'Stop.. throwing.. stones... into.. the... pond!!!!!
If the mom or brother protest at you shouting at the kids, say'Well I'm sorry but I gave you the chance to discipline your own children and you didn't'. And then leave it at that. Don't say anymore. Cheeky sods.