I stumbled on a UN page on a claim that 40 % of all child soldiers are girls. Now, that can't be true if talk about combat soldiers but I didn't manage to find a more realistic figure.

It seems that "child soldiers" are "children associated with armed forces and armed groups". So, the term is perhaps a bit misleading though I understand that there are all kinds of tasks in an army and not everybody fights. But it seems that female child soldiers don't necessarily have any military purpose.

My guess is that the UN and others wanted to make this also or perhaps predominantly a "girls' problem" and started counting others than just soldiers into the category of soldiers. Knowing something about the track record of the organization, that doesn't sound too implausible.

Anyway, I also guess that of the actual child soldiers, those who face the enemy and aim to kill, more than 90 % are boys. But didn't manage to find a reliable estimate.