One of the easiest ways to bring vintage into a child's bedroom would have to be framed pages of a vintage story book. I used these pages out of a Fun to Learn Book published in 1983. I picked it up at the local op shop for $1 but what I didn't know was that half the pages had been attacked with felt tips. Nevermind, the story about Peter's house went untouched and I think it makes a nice set of pictures for Elijah's room.
So next time you need some cheap art grab a few frames and a secondhand book - takes all of 5 minutes for a set of three (A$ size)