Everyone has excellent points here, and I think the solution is simply to choose, since we determine what is appropriate for Mordor's inhabitants at this time in ME history.
Trust me, I can be quite the feminist, but I'm also often a stick-in-the-mud realist when it comes to roleplaying and storytelling in general - and right now I'm just trying to figure out what could be 'realistic' in the Fourth Age of ME Mordor.
At any rate, perhaps the wisest way to go is to say that all of the slaves will play a role, all will be armed with whatever they can find, but, the older slaves, the women who are not accustomed to fighting or would not like to fight (or who simply volunteer to look after and guard the children), and the younger children will be in charge of the traps and decoys we must plan. The men, women who are quite fine wielding a weapon, and the older children (which could range from even 13 or 14 on), will be taking up the 'better' weapons available, and being the core of the slave fighting force.
What those numbers are is relative, I think. But I think it is safe to assume that the majority of the slaves will be fighting. We could make it simple and approximate 40:20, or even 45:15, to get an idea.
Does anyone (or everyone) feel we need specific numbers? I don't know if that's quite important.