I use Revlon Color Silk. It's dirt cheap ($2.50 - $3.00) and easy to use. The trick is to leave it on long enough to get the gray covered without keeping it on so long that the color looks artificial. You want some of the hair's natural translucence to show through. So follow package instructions conservatively, and if you miss some gray, touch it up.
That red-headed baby is really cute, even with the bad hair. What mother would style her hair like that? And where do you find these pictures?