Pre-stretched canvas We used 16"x 20," but they come in a variety of sizes. I got 2 for $5.99 on sale.
A set of brushes in a variety of sizes (I purchased a rather complete set that included a roll-up canvas storage case for $4.99 on sale. You can get smaller sets for $2.99).
Newspaper to protect your work surfaces.
A surface/container on which to mix paint. We use tin foil.
A small container for water. We use margarine or cream cheese containers.
Smocks to protect your kids' clothes. We use old adult sized T-shirts- they cover everything!