Answer:
A hole leading in, a hole leading out, we connect to a cavern that is slimy all throughout. What are we?
Upon me you can tread, though softly under cover. And I will take you places, that you have yet to discover. I'm high, and I'm low, though flat in the middle. And though a joy to the children, adults think of me little.
The moon is my father. The sea is my mother. I have a million brothers. I die when I reach land.
When it is alive we sing, when it is dead we clap our hands. What is it?
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.
People take me on walks and I run and have fun. What am I?