It's not easy telling the kids, but they are amazingly resislient.
Each of our children were told when they were around 10. My daughter because she seem to be at an age that she could handle some of the basics and understand about not telling just anyone. My son, I didn't feel was quite as prepared, but at the time my husband was in the hospital and not expected to live and he needed information.
One reason I feel it's important to tell the children is from a health standpoint. It's OK to teach them universal precautions, but in the event something should happen to you and you are bleeding and can't take care of yourself, they need to be aware of your status and you might be unable to tell them at that point.
Kid's see and know more than we think they do. It may turn out that the one that is surprised is you. When you do plan to talk to them. Make sure it's a time that you won't be rushed or interrupted. They may or may not have questions right away. They may have questions later.
My daughter is now 22 and my son is 20. They've grown up knowing mom and pop have HIV. I think it's given them the opportunity to become better educated about HIV than most kids and that information they can impart to their friends. It made them both aware of the importance of personal responsbility when having sex.