Recommended Arts and Crafts Supplies
Watercolor
Brushes
You can find affordable craft brushes that are marked for watercolor or watercolor/acrylic in crafts stores. Watercolor/acrylic brushes are softer than oil/acrylic brushes. Well over half of the brushes I use for my watercolors are craft brushes.
Watercolor Paint
For younger children, pan watercolors (the dried ovals of paint that come on a white tray inside a transparent box) work well. I prefer Crayola watercolors over Rose Art watercolors: Crayola seems to have less fillers and more pigment in their paint.
Older children will have better success with tube paints. Student grade watercolors have more filler and are less light-fast than artist grade watercolors, but student grade watercolors are a great introduction to the different colors. If you have a truly artistic teen on your hands, consider buying three basic colors (Lemon Yellow, Permanent Rose, and Prussian Blue) that they use to learn mixing almost any color they need.
Watercolor Paper
If you can splurge on only one material, choose good quality watercolor paper over the brushes and the paint. You can get beautiful results with inexpensive brushes and paint as long as the paper allows the paint to move the way it should. Watercolor paper comes in three different surfaces.