Here are a few art supply recommendations for children.
Make sure to invest in some good quality brushes, even for children. I recommend the Round Tip brushes. Quality brushes will help children (and adults) have better control of the paint, therefore leading to a more rewarding painting/art session and experience.
All of these will link to Amazon.com – I find this helpful to place what I need or want into the cart, then I can see exactly how much everything will cost and can decide from there which products to get. Hope this is helpful!