It is difficult to be sure until they are a bit bigger and/or clearer to see in a photo (looking at legs arrangementto differentiate between lepidoptera na sawfly, for instance).
But if I had to have a stab now, I'd say a type of sawfly (many of them have black heads and grey bodies).
Thanks - they get busy, don't they?!
Give it a few days and you might be able to recognize if they are sawfly larvae by their characteristic pose (as well as the legs arrangement); or not.