I've always pretty much taken the same stance as you in that if they are ready to ask the question, then they are ready to hear the answer, but it doesnt mean they want to hear all about sexual intercourse. A simple answer to the question was usually enough and if they wanted further explanation, my girls would ask. As they got older, they asked more questions.
I can understand why some of the parents would be concerned with someone outside the family teaching their children about procreation, after all, they don't know if they'll be teaching things that they may feel inappropriate, especially as we hear more stories of child grooming. It sounds like you handled their concerns well and allayed their fears, though.