Answer:
Canine children.
A little pool with two layers of wall around it. One white and soft and the other dark and hard, amidst a light brown grassy lawn with an outline of a green grass. What am I?
I walk on 4 legs in the morning, 2 legs at noon, and 3 legs at night. What am I?
Drop me and the world will shatter. You can always find me where you empty your bladder.
When it comes to me you go on red and stop on green.
No matter how little or how much you use me, you change me every month.