So in grade 4 my teacher read us a book that I ended up really liking but years later I can't remember the name of it what so ever and it has been literally bugging me for years and I've even tried to research it and have come up with nothing. But I do remember a few details of the book:
There was this boy who was playing with a ball and it accidentally went down a drain or pipe (not 100% sure). So he goes down this drain or pipe and after a while it opens up into a massive cavern/cave sort of area. And in the middle, on a large pile was a nest and right at the top there was a rainbow dragon egg. A few other things happen that I can't remember but the boy ends up befriending the mother dragon and he starts visiting her daily. Then at one point the boy discovers the area where the dragon is living is going to be demolished within a few weeks (I think). Then all this stuff that I also can't remember happening happens.
The mother dragon can't leave because her egg hasn't hatched yet and then soon after it does and the boy gets to meet the baby dragon. Then when the dragons leaves, she gives the boy a scale, jewel or piece of the broke egg (once again I'm not sure) and then after they leave he discovers that he can see the dragon through the object she gave him.