Easy:
Country Concerto by Howard Kasschau
elementary, easy
A really fun piece for little kids

Medium:
Concerto in G major by Robert Vandall (Bradley Publications)
intermediate/medium difficulty
We all know and love Vandall. This is a very attractive piece, with overtones of Kabalevsky.

Miniature Concerto by Alec Rowley (Boosey & Hawkes)
intermediate/medium difficulty
Lots of fun.

Concerto in C major by Haydn (Boosey & Hawkes)
elementary/medium difficulty.

Competition Level:
Concerto in D major by Haydn. 1st or 3rd movements
intermediate (but on the difficult side)

Concerti in F major, K. 413, and A major, K. 414 by Mozart
also intermediate but more difficult than the Haydn D major

Concerto in A major, K. 488 by Mozart
advanced musically but not particularly difficult technically

Concerti in C major, Op. 15 (last movement); B flat major, Op. 19
(1st and 3rd movements) by Beethoven

"Youth" Concerto, Op. 50, No. 3 by Kabalevsky
not too demanding technically; good rhythm required

Concerto in c minor, Op. 37 by Beethoven, 1st movement
advanced

Concerto in g minor, Op. 25, 1st or 3rd movements by Mendelssohn
advanced

Concerto in a minor, Op. 16, by Grieg
advanced

Concerto in a minor, Op. 54, by Schumann
advanced

Concerto No. 2 in g minor, Op. 22, by Saint-Saens
advanced