young, weak
嫩 centers on the idea of something young and therefore tender or weak, like a new plant or a child. The two given meanings are closely connected: youth implies a lack of strength or resilience.
嫩 combines 女 (woman) with 敕, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with youth and tenderness.
A woman 女 beside a complex shape that looks like a bundle of fresh, young sprouts: the whole character suggests something young and tender, easily bent or weak.
For ドン, imagine a young, weak seedling that goes 'don' when it falls over in the wind: the soft thud 'don' cues ドン, and the seedling is young and weak.
bud; sprout