How is the Wood between the Worlds like the tunnel in the attic?   

They think the wood like the tunnel. The Wood is so quiet and sleepy like tunnel in the attic. Like in the tunnel, nothing goes on in the in-between places. But when you come out of the tunnel , you will go back to your world. There is nothing to happen in the Wood too. And there are many pools, maybe each pool is a door to a world.

Like the tunnel is not belong to every house, the Wood is too and  it's  just a sort of in-between place between each world.

The similarity between the two places gives the boy a wild idea. What is it?  

the boy thinks that if they can get back to their own world by jumping into one pool, Maybe they can get somewhere by jumping into the others. They suppose there was a world at the bottom of each pool. So he would like to have fun instead of getting caught by Uncle Andrew and taking away rings.