Mozart's variations on the Dutch patriotic song 'Willem van Nassau' are a charming example of his early compositional skill. The variations are generally lighthearted and demonstrate his ingenious ability to re-imagine a simple melody.
Focus on crisp articulation and precise rhythmic execution, particularly in the faster-paced variations.
7 Variations on 'Willem von Nassau' (K.25) is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 5.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 7 Variations on 'Willem von Nassau' (K.25) lasts about 6 minutes.
7 Variations on 'Willem von Nassau' (K.25) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.