According to Serenes Forest, children inherit the one skill from each parent that is equipped by the parent and is listed in the last position of their skill list. The child inherits those skills the moment one begins the chapter in which he/she is recruited.
I had paired up Donny with Lissa and just started the chapter, in which Owain is recruited and - stupidly - forgot to reorder Donny's and Lissa's skills beforehand, resulting in Owain not inheriting Donny's Aptitude and inheriting Healtouch from his mother which is useless for a non-healer.
Even worse: I noticed my blunder only after I had saved in battle-preparation mode right before beginning battle.
Unsurprisingly, reordering the parents' skills during battle-preparation is already too late.
Is it possible to reskill the child, without having to restart a new game?