Using the coloring pages
The short version: print as much as you like for children, do not resell the drawings.
At home and in a classroom
Print whatever you need, as often as you need. A teacher may print a drawing for a whole class, a childminder for the group, a parent for a rainy afternoon. No permission needed and no attribution required.
At a party or an event
A birthday, a school fair, a waiting room, a hospital ward: all fine. You may hand the printed sheets out. What you may not do is charge for them, or hand them out as part of something people paid for.
What is not allowed
Reselling the drawings, in print or as files. Putting them on another site, with or without the watermark removed. Bundling them into a product, a book or an app. The drawings are free to use, not free to redistribute.