Realise you left this post a while ago, but I've just seen it. I know what you mean about the mess.. but I don't think anyone clears it up.. I think just the rain washes it away. We seem to get enough rain for that..
I think it's nice to see the kids feeding the birds it's a fun free thing for them to do. I expect if it becomes too bad with getting attacked by the birds something will have to be done. For now I'm happy to just watch where I walk.
Regarding the birds-what type of birds? Are we talking seagulls or pigeons.? In some places it is an offense to feed the pigeons. If it is seagulls well guess what? We live by the sea! What makes it all worse is litter from takeaways and the birds do pull it out of the bins! Seagulls and crows are very good at pulling carriers out of bins and scavenging. This is partly what encourages the birds to that part of town. It is nice to see children feeding them but it has the downside of helping them breed, encouraging them to return to the area. The damage they do with their droppings is to the buildings as it is acidic. The rain does indeed wash away anything at ground level. Myself I just think it is part of the package of living near the sea so I never complain about it.