I can't remember if i've talked about this specific subject with my son (he's 11)...but i dont find it difficult to talk about issues from "both sides" of a controversy, without necessarily injecting my own opinion about it. Obviously, i share my opinions about issues with him, but i try hard to give many points of view. I dont think its right to act as if there arent good points to both sides of an issue...just because i might not agree with the point, doesnt necessarily mean the other side is "wrong", they just think differently than i do. (Therefore, i wouldnt tell my son something so horrible as women who have abortions are "not nice", when that is simply not true. In fact, i'm betting that plenty of "nice" women you know have had one.)
So...i might tell my son something like this: "Abortion is when a pregnancy ends before a baby is born. It can be something that happens naturally, or it can be a medical procedure that the woman chooses to have done, either through surgery, or with a pill. Usually, these procedures are done very early in the pregnancy, before the fetus/baby is able to live on its own. Sometimes its done later, often for reasons like the woman's health being in danger, or the baby being sick and unlikely to live a "normal" life. Some people think a woman should be able to make the choice to have an abortion, and thats called being "prochoice". Some people think that the woman should not be able to do it, that a developing fetus is a person from the beginning. They consider it killing a human person, and they are called "prolife"."
I might go into some of the historical facts, like about Roe vs. Wade, illegal abortions, etc. I might talk about my experience with escorting at abortion clinics when i was a teenager. I might talk about my discomfort with "abortion for fetal anomaly", and the societal implications i believe arise from that (or perhaps are symtoms of.) But i would also discuss why someone might make that difficult choice. I would likely talk about why someone might be prochoice or prolife, and perhaps talk about what each group might have in common with the other. I might talk about how many many women in this country have had abortions, and that such women come from all socioeconomic levels, races, ages, etc. That they may choose to terminate for many different reasons, and that some would perhaps choose to continue if they had support (either from family or economic support)...i might talk about ways we as a society could reduce the number of unwanted pregnancies, and also ways our society could function that could help ensure no woman was coerced to abort or to remain pregnant. I might talk about the other options...parenting, adoption...and the possible pros/cons of each.
My goal would not be to make sure my child thinks just like me, but that he has a good grasp of all the many issues surrounding this one big issue, so that he can decide for himself how he feels (and how he feels can change over time.) I would probably not discuss this issue all in one conversation, but add more to it over time.
My son is 11, and i obviously would not have such an indepth conversation with a five yr old (and probably would start a conversation with "what do *you* think abortion is? Where did you hear it?") It made me sad when i would see little children at protests, holding very graphic pictures (on both sides of the issue). I think there is a way to satisfy a child's curiousity about a word they do not know, without laying out all the graphic and adult details.