IIRC if there was a situation in form of "3 children on road" and "1 old lady on the sidewalk" and you only had a couple seconds to react, you would be fine hitting either, there is a concept of "Übergesetzlicher Notstand" (emergency beyond the law) which hits exactly these rather rare and morally ambiguous situations.

The trolley problem as bystander is a different thing and generally not acting is the legally safer option though in both cases the likely sentence would be minor through various means.

edit/additional note: while you can't shoot down the plane legally and no soldier will be obligated to follow such an order, they would likely also fall under the "Übergesetzlicher Notstand". It's not a legal basis but simply a defense.