Picture book (circa 1970s) in which a small child who lives in a Manhattan apartment house manages to get on the elevator on his tricycle unaccompanied, and gets out onto the street to explore his neighborhood.
Stump the Bookseller
New format, same book stumper solving.
Sounds a bit like Tell Me a Mitzi, but no tricycle in that one.
This doesn’t match your plot exactly, but you might look at Tell Me a Mitzi, by Lore Segal, illustrated by Harriet Pincus. A compilation of three stories, and in one, a young girl takes her toddler brother outside in his stroller. You can see reproductions and a plot summary here:
http://www.vintagechildrensbooksmykidloves.com/2010/01/blog-post_07.html