ShiroKuro
Difficulty is very subjective so take this with a grain of salt but here is my general take on sonatinas and easy sonatas:
Easy (approx. RCM 3-5)
Beethoven Sonatina in G major, Anh. 5, no. 1
Beethoven Sonatina in F major, Anh. 5, no. 2
Clementi Sonatina in C major, op. 36, no. 1
Clementi Sonatina in G major, op. 36, no. 2
Diabelli Sonatina in C major, op. 168, no. 3
Diabelli Sonatina in G major, op. 151, no. 1
Haydn Piano Sonata G major Hob. XVI:8
Kuhlau Sonatina in C major, op. 55, no. 1
Lichner Sonatina in C major, op. 4, no. 1
Lichner Sonatina in C major, op. 66, no. 1
Lichner Sonatina in F major, op. 66, no. 2
Lichner Sonatina in G major, op. 66, no. 3
Reinecke Sonatina in C major, op. 136, no. 1
Reinecke Sonatina in G major, op. 136, no. 2
Spindler Sonatina in C major, op. 157, no. 4
Spindler Sonatina in E minor, op. 157, no. 8
Medium (approx. RCM 6-7)
Clementi Sonatina in C major, op. 36, no. 3
Clementi Sonatina in F major, op. 36, no. 4
Clementi Sonatina in G major, op. 36, no. 5
Clementi Sonatina in D major, op. 36, no. 6
Diabelli Sonatina in C major, op. 151, no. 4
Dussek Sonatina in G major, op. 20, no. 1
Dussek Sonatina in F major, op. 20, no. 3
Dussek Sonatina in E-flat major, op. 20, no. 6
Haydn Piano Sonata C major Hob. XVI:1
Haydn Piano Sonata C major Hob. XVI:3
Haydn Piano Sonata D major Hob. XVI:4
Haydn Piano Sonata D major Hob. XVI:5a
Haydn Piano Sonata C major Hob. XVI:7
Haydn Piano Sonata F major Hob. XVI:9
Haydn Piano Sonata C major Hob. XVI:10
Haydn Piano Sonata G major Hob. XVI:11
Haydn Piano Sonata E major Hob. XVI:13
Haydn Piano Sonata D major Hob. XVII: D1
Haydn Piano Sonata G major Hob. XVI:G1
Kuhlau Sonatina in C major, op. 20, no. 1
Kuhlau Sonatina in G major, op. 55, no. 2
Kuhlau Sonatina in C major, op. 55, no. 3
Mozart Viennese Sonatina no. 1 in C major
Mozart Viennese Sonatina no. 2 in A major
Mozart Viennese Sonatina no. 3 in D major
Mozart Viennese Sonatina no. 4 in B-flat major
Mozart Viennese Sonatina no. 5 in F major
Mozart Viennese Sonatina no. 6 in C major
Difficult (RCM 8 and higher)
Bartók Sonatina, Sz. 55
Beethoven Sonatina in C major, WoO 51
Beethoven Sonata no. 19 in G minor, op. 49, no. 1
Beethoven Sonata no. 20 in G major, op. 49, no. 2
Kuhlau Sonatina in G major, op. 20, no. 2
Mozart Sonata no. 16 in C major, K. 545
The "difficult" section is relative to the rest of course. After this you start getting into real "grown up" sonatas.