A head-on collision would be when two cars crash while driving directly toward one another. It wouldn't generally be used for people bumping into one another. In your first example, I would say "they accidentally bumped heads." Your second sentence is grammatically correct, but unclear what it refers to.

A good alternative way to say it would be a "clash of heads"

Unfortunately head-on collision just means they were both facing each other when they collided, and out of context usually only works with vehicles (or if we know the two people are walking towards each other etc). I don't think there is a term in English that describes this exact thing. For both examples, the best I can think of is to say "Their heads accidentally collided" and "She remembered when their heads had collided earlier" The second example isn't 'wrong' (and actually is much more interesting than my rephrase) but right now it could just mean that they walked into each other face first. It depends how much detail you want to go into :)