I teach sex ed and the best response is an honest one. Use correct terms (or terms you use as a family), keep it simple, uncomplicated and factual. It of course depends on the age and maturity of the child asking, keep it at their level. If you make things up or brush it off, it only adds to their confusion and/or curiousity - who knows where they go to find out then!
We actually use the Where Did I Come From video for our year 7 students. It is simple to understand and because it is like a cartoon, kids like it. The 11 and 12 year olds at first think it's a bit babyish, but then they start learning things and are the quietest they will be all year. It would be ok for a 5 yo, but probably a quick factual answer will be enough - for now. Too much info might just get confusing. Good luck.

Just remember - show no fear or embarassment. Kids (as you would know) sense it and will either ask more to find out why your'e like it or back off because they think they have done something wrong.