Start being nice to them. Invite them over one night (I mean night). Before they arrive, make slender pages and put them around an area of the house where your parents aren't (basement, attic etc.) Download slender, let them play it. Hide the pages but try to do it as quietly as possible. When they get caught, make sure the lights go out. Throw tennis balls or ripped teddy bear heads where they are, freak them out. Then finally, get a small table, stand on it (wearing a long jacket) and wait for them to turn around. They'll sh/t themselves.
EDIT: Wait up...they're 8 and 10? Wth, just ignore them. If your being bullied by people who are that young...wow dude. Just wow.