Because the wet paint is laid on wet paper, the watercolors are flowing and blending into one another in beautiful, unexpected ways. The use of watercolors allows fluid mixing and movement on the paper.
My experience is that painting with watercolors is a very relaxing and calming activity. To invite creativity it is helpful to set up everything You need in a quiet atmosphere. All gestures of Your preparation should be performed mindful. 
To get started You will need:
A sink, basin or tray in which to soak your paper. We use a normal baking tray placed on the table for easy access.
Heavyweight or normal watercolor paper, heavyweight paper is more expensive but the result is worth it.
Some shades of watercolor paint. We use an recommend Stockmar paint
Carmine Red, Ultramarine Blue and Lemon Yellow is all You need for the beginning.
A watercolor paintbrush with flat bristles like this one.
A jar for water
One little jar for each color of paint
A flat board on which to work like this one.
A sponge like this one and a little rag
1. Soak one sheet of paper at a time in the water You have prepared. The paper can soak about 10 minutes.
2. Mix Your colors with water, add more paint to make it stronger, or more water to soften.
3. Lay a piece of soaked paper on a flat board. Watercolor paper has a rougher side and a smoother side. Lay paper down with rough side up.
4. Use a wet sponge to make sure there are no puddles of water on the paper, and wipe away any air bubbles. Paper should have a sheen to it, but not be soaking wet.
Start with just one color at a time. After painting with each of the primary colors singly, try the two color combinations: red and yellow, red and blue, and yellow and blue. Children will be excited as they experience how secondary colors are formed when combining two colors.
Let Your child experiment with the colors one by one, there is no need to explain to much. Just sit down with Your child and start painting and be a good example. Childen find out and internalize soon, that yellow and red will make a beautiful orange and that red and blue can be all shades of violet.
