There are online trip insurance comparison sites -- I believe you can purchase insurance up until fairly close to your departure. Just know that anything that happens before you purchase insurance will not be covered (named storms, illness/injury, etc.). Are you concerned about having to cancel if 1 child gets the flu? Or could that child stay home with the parents and you take the other 2 on the trip? Anybody can get sick regardless of receiving the flu shot, and if someone comes down with something while on vacation I recommend that you have copies of the kids' insurance cards with you.
Enjoy your vacation!