I think one of the most important things you can do to encourage your artist is to invest in better quality for their art supplies. I don't think you need to spend a lot, but at least get them supplies that can encourage better results.
These are my tips for getting your artist set up properly with watercolors. (the photo is of my 3 year old daughter's set up)
Honestly, I think anyone would be frustrated and loose interest if the color they dip their brush into a color that is only capable of giving an anemic version of what it looks like. I think that you should spend about $10 on a watercolor set (or get one of those 40% off coupons and get an even nicer one). Brand doesn't necessarily matter as much as when you start spending that much, you will be getting more pigment in the paint, hence better results, and the set is going to last a lot longer. I also think it is important to get a variety of brushes so that your artist has choices. There are typically variety packs of synthetic brushes available at most art and craft stores for 5 or 6 dollars. And finally, paper- if you give your artist printer paper to paint on, it warps and tears- it is not made to accept water. I recommend buying a ream of card stock (stores like xpedex have great prices and selection) You can get something with a little texture, or if you want smooth- I suggest "bristol" card stock.
Better quality supplies=better art. Your artists deserves it!