I am the eldest in my family, so have seen how the toing and froing of relations change and alter in some unknown way, often without a specific reason. One day best buddies playing happily together and the following day endless quarrels etc. So I do think there has to be a clear divide, between general argy bargy and on the other hand bullying or certainly throwing stones or anything that could cause harm to others. Whilst the parent really have to have clear rules as to the behaviour expected, I would suggest that as a grandparent, you have every right to have your own rules in your house. Any major problems would result in sanctions and I would suggest that the simple statement that you are unable to take them to swimming or park or whatever if their behaviour is such that it is either not safe for themselves and others, or they overstep the mark in your house. If they continue, then I do think that you need to talk to their parents, explaining that as things stand you do not feel that they accept rules or your right to decide what is going to happen, so that you are unable to look after them at this time. You are not saying this in an accusitory way, the family has the right to decide what is acceptable behaviour in their home, but you also have the right to say that if you are not going to be in charge, then it can feel very worrying or even a little dangerous. As an adult you see the dangers in a situation and if they are not going to listen when you say you need to move from the edge or whatever , then it really is not safe for them or you to carry on in that way. To me the only thing that could work would be that you could take them to the swimming pool or a football match in a park or whatever , where someone else is running the class but you are there to take them and to encourage and praise them, but are now only expected to be an onlooker and not have to manage their behaviour.