The pitch is between 2nd line G (which may be too high for them initially)and low C (which may encourage them to NOT firm their corners and flatten their bottom lip enough early on).
Additionally, E is the first pitch of Hot Cross Buns, Mary Had a Little Lamb, and many other simple melodies we buzz early on in the year.
Also the trumpet kids want to press the valves so playing E allows for this.
We learn E, D then C (buzzing and playing).
Then we learn 2nd line G once we're great at the first three notes. Then we finally add F at which time we are able to play "down the horn" G F D D C.
Then add music (the visual component to playing) and we're off to the races!