My boys have it available to them at all times, finally they are starting to pick up after themselves too!!

Lyra pencils -we use the ferby- I don't know what we ever did without them. Both of my DS (7&3) use these EVERY day. They are worth every cent, and last longer than you think they will.
Good quality Tempera paints. I got some Urecht studio series tempera paints that Lilian suggested (she has a lot of great resources). I bought a bunch because they were on sale, but they are not that expensive even at full price. Night and day difference from the store-brand "poster paint." So glad I made the switch.
Those are my two favorites. Other supplies they use often that are appropriate size for them.
-tape
-scissors
-small stapler(you'd be amazed what they can create with this!)
-paper
-crayon carosel - I have stockmar crayons & block but they still prefer crayola.
- various texture brushes
-a few good quality brushes (this does make all the differnce)
-markers
These are the things that they use on a daily basis. I also have stockmar watercolor paint and board but I'm not quite sure how to use it yet. Looks like it will be a beautiful experience for the boys.
Great thread! Curious as to other peoples must haves. Maybe Lilian will drop in with some great suggestions!