Why would Morpheus give Nemo the key?

I guess when he was going to make Nemo his heir, he wanted him to have the key to keep the Nightmare King locked in the underground chamber, but at the same time, I think giving him the key was a big mistake with how Nemo's young curiosity and Flip's child-like irresponsibility get in the way and he opens the door anyway. He told Nemo he could open an door with that key, but seriously, what doors in Slumberland are that necessary to open with that key? It's like the key was made specifically to keep the Nightmare trapped and in giving it to Nemo, he was leaving him with the power to open it. It's as though Morpheus was testing Nemo; he never told Nemo what was behind the door, only fiercely telling him not to open it, which only left Nemo with enough curiosity to open it with Flip's help.