Can you feel the bright sun and warmth of the day, when I saw the scene at a petting zoo in Oregon? I will tell you how I created the glow. You first treat the Strathmore Aquarius paper with gloss medium, which makes it slick and nonabsorbent. (Aquarius is the part-synthetic paper which, although light-weight, doesn't buckle when wet.) Paints puddle and float; so let them mix on their own and dry naturally.
The beauty of the technique is that paints can be easily lifted to reveal the glowing, pure white of the paper. But painting on such a treated surface is also tricky, because you can inadvertently lift the previous layer while glazing--use a light touch!
I like to use granular paints like French Ultramarine and Holbein's Mineral Violet; you can see the effect of granulation clearly in the background. After it dried completely overnight, I lifted the sun-struck areas and painted the boy who is feeding a pony and three goats.
"Kids" was published in How Did You Paint That?: 100 Ways to Paint People and Figures in 2003. It also won an honorable mention in The Art League show in 2003 and an honorable mention in the Potomac Valley Watercolorists Invitational Show in 2004.