The Nick Jr Coloring book has all it needs for online coloring for your kid. It has tools like crayon, spray can, paint bucket, 3 brush sizes, eraser, several coloring pages accessible on the same screen, and a full color pallet. This should be your first choice for online painting.
This image has prominently sketched lines, easy for kids to fill the color in.
Mushrooms from woods
The sea shell shelf
This page allows you to add colors and animated objects into the pictures. More fun than static color pages.