Two kids are going to see what's inside of a tunnel.
And when they arrive at the place one of them says: 'That's good'
and they stop riding their bikes. Does it mean something like 'this is the spot'?
Funny expression to use there, but yes, that's about it. I've usually heard this to mean, "That's enough."

Can I have just a little gravy on my mashed potatoes? (Mom ladles gravy) That's good, thanks.

I mean, I've heard it used in this context to mean "that's enough." Obviously "that's good" can have a wider range of meaning Emotion: smile
Thanks, D!