Problem with my neighbour...is there anything I can do? (UK)?

Unless there is actual proof that your neighbor's kids are doing this, then there isn't much you can do against her. Security cameras are a good idea for gaining proof. If this is happening to many other people in the neighborhood then you can confront her with others as you might not want to do it by yourself. Someone suggested moving and that is always an option though I get the feeling you don't want to. Basically either you get proof or you confront her, though without proof she will probably not listen about your kids and be offended and could possibly react negatively toward you.