A top level event is one that's not a sub-event of anything. There's another thread here where R0J0hound links to a picture from CC that explains it.
It depends what exactly you're trying to do. You could either do whatever you're doing in the for each loop inside the while loop instead, immediately after the sprite is spawned. Or inside an 'on created' trigger. Or you could set a variable true when the sprites are spawned and have a top level event 'var = true/ for each sprite' -> do stuff.