We also have the expression “le cadet de mes soucis”

The least of my mice? D:
(because I certainly don't want mice running around my place)

(j/k, kidding)

Yeah, and I also forgot from my comment above that there is indeed the "cadet-branch" in terms of lineage records, something which tended to be important for the 'nobility.'

Merci !! Je n'en avais jamais tant appris :)))