The little girl bears a passing resemblance to the character of Madeline, from the children's book by Ludwig Bemelmans. It was first published in 1939.
Background detail was becoming more modern and abstract, as shown here, with the advent of mid century design.
The little red wagon that Junior has was a popular toy for many decades. Modern versions exist today, but an original restored model is worth quite a bit of money to collectors now.
Rabies in dogs was still a health concern at the time of this short. Modern vaccines have reduced the threat, though the disease continues to be found in bats, skunks, squirrels and other creatures. The sight of a dog foaming at the mouth in 1958 would have created quite a calamitous scene.
Junior says, "I wonder what's goin' on behind the green door." This refers to the popular 1956 Jim Lowe song, "Green Door."