The Merriam Webster dictionary calls a ditty an especially simple and unaffected song

But simple is hard. This was known from the medieval period to von Clausewitz.

A Ditty –

My true-love hath my heart, and I have his,
By just exchange one to the other given:
I hold his dear, and mine he cannot miss,
There never was a better bargain driven:
My true-love hath my heart, and I have his.

His heart in me keeps him and me in one,
My heart in him his thoughts and senses guides:
He loves my heart, for once it was his own,
I cherish his because in me it bides:
My true-love hath my heart, and I have his.

Sir Philip Sidney