Do nothing. Whilst I dislike tailgating, we've all done it to an extent when we know the road and the guy in front is dawdling. Now I'm not saying you were (although "taking it pretty easy" does give that impression), but tbh there was no harm done and you moved over to let him past. Better to have the idiot in front than behind etc.
There's already too many people sending footage in to the police when really it's a bunch of nothing and no harm was done. You even say yourself you've no idea if there were kids in it or not, tbh it's not difficult to see a bunch of kids in a bus when it goes past you so I'm willing to bet it was empty. Move on, life's too short.