as a former preschool teacher and now mom of 3 I've gone through my fair share of art supplies, these are ones we rebuy time and time again. Tried and True favorites!
dot markers are a must in our house
Do A Dot Art! Markers 6-Pack Rainbow Washable
we go through a lot of paint in our house, this paint is washable and comes out of everything (not that I have any experience with this, haha kids)
Tempera Paint
paper for the easel or just to tape to the table is always handy
Roll Of Paper
pro tip (spray the color with a water before you give to the kids and you won't have to worry about them pouring water)