I know that had to be aggravating. But I would not go so far as to say "stupid children." Since grandpa was just as rude, I would say "Poorly-raised children." If something like that happens, you can always use it as a teaching experience. Let them know nicely that this is a living creature just like them and they should not tease or be mean; that if they treat them nicely, pets can be such a joy. They may still be rude, but at least you have planted a seed. Sounds like those kids have not had much sowing of good will towards animals.
Easy to say after the fact, I know. My first impulse might not be as nice.
