You will want to keep these supplies on hand at all times as you can complete at least half of our projects with these supplies alone.
Now there will be other supplies each month that are activity specific, but I will try to keep them to a minimum.
My one last suggestion regarding supplies: become a crazy person! Keep all toilet paper rolls, boxes, bottles, and bubble wraps since they are all AWESOME materials to use with Lil’s. He who throws away a toilet paper roll in my house is looking for a fight. Some call it crazy, but I call it being frugal and resourceful.