I think it's a great idea to pop a bed in the nursery, wish we had done the same as it would have saved us laying on the floor! Is a king single longer than a single? I can quite comfortably curl up with our 3 year old in his bed but my DH finds it way too uncomfortable. He is 6 foot and solid build if that helps.
Yep this even down to the size of DH

Eta: a friend of mine had a single in her nursery with the cot and at first I was like "Whaaa?" But now I get it.