Ridiculous as it may sound, nowadays especially I would imagine that going out and telling youngsters off can be very risky to the person doing so, not all children are receptive to threats or even being told off and sadly can in some cases end up making life more difficult for the tenant involved. Better surely to first try flattery. Pick out the hardest or the worst one of the crowd of kids, take him to one side and say something like, "look, you seem like a really nice lad (or girl), could I ask you a big favour? there is an elderly lady not very well at all living in the house/ground floor flat where you are kicking the ball against the wall. I would really appreciate it if you could get the others to stop doing it as it is upsetting her". Kid's can respond positively if they are spoken to kindly and he/she can be seen as a sensible "hero". It may not work of course but surely this is better than confrontation from the beginning. If it doesn't work then maybe you know lads/lasses older than those being a nuisance to have a quiet word with them on your behalf. Just a thought.