Since everything a toddler does become an invigorating learning experience, the images should definitely be of things that they recall, if they can name it, the better. Toddlers learn better through visual examples and what better way to do this if not with coloring pages. Hence, you can use coloring books to teach more to the toddlers than just colors, you can do the alphabet or count numbers. The alphabet or numbers can also be big enough for the children to do some coloring on them too.
He will soon be old enough that in a year or so we will be able to break out the paints and see what he is like using them on the Coloring Pages. I know he will love this although I am already nervous at the idea of the chaos it could make! I am sure his eyes will go wide the first time I mix two colors together to make a brand new one, such as red and yellow combining to make orange. It is so rewarding to see his progress over time as he learns without knowing he is doing so. I can really see how his ability to concentrate has improved.
Coloring pages for girls will allow your little princesses to show their flair and creative powers in many ways. While young they may not make the most beautiful creation and they may not color inside the lines all the time but they will color some amazing things (in their eyes). As they grow older they will begin to desire to color inside the lines and to combine the proper colors to make some truly wonderful pictures that you will be proud to display and may even want to frame.