I'm sorry, pal, but unless you want to be a 30 something grandparent, then the only real way you will keep your daughters safe is to teach them to abstain, and to prohibit them from dating or being around boys altogether except in school or at church. Abstinence is the only 100% sure method of contraception- every other method has at least a slight risk of failure, and for some of them, it is more than slight.
As an alternative, have you thought of putting both girls on the pill? When taken correctly, the birth control pill is very effective and reliable- and contrary to popular opinion, it is actually quite safe. The dosage of hormones used in today's pills is actually much lower than what was used when the BC pill was invented in the '60's, which means your daughters' risk of developing side effects, or of contracting cancer, are greatly reduced now. IUD's are dangerous in girls this age, because they can and often do cause "silent" infections in the reproductive tract, which in turn can cause permanent scarring and infertility later in life. Somehow, I can't imagine that in your quest to keep from becoming a grandma prematurely, you would want something like that to happen to either of your girls. If it did, it would mean that you would likely NEVER be able to enjoy grandchildren, unless one of the girls adopted a child.
Other than suggesting that you put the girls on the pill and teach them to abstain, there are no other ideas I can think of. You are right- kids today will have sex, no matter what their parents think, just because they can. That's sad, and I wish it were not true, but hey, there's really nothing anyone can do about it. All the preaching in the world doesn't make any difference when kids have celebrities like Jamie Lynn Spears as role models, or athletes like LeBron James as sports heroes ( he got his girl friend pregnant and became a daddy at 19, sans wedding ring, and Time Magazine chose him as a role model despite this. What does that say about our society as a whole?) who are constantly touting the idea that premarital sex is an acceptable thing to do. Even the "Brangelina" couple can hardly be considered appropriate role models for today's teens- THEY AREN'T even MARRIED, for Heaven's sake, and they have 6 kids now !! In short, your girls are getting bombarded with messages about sex daily, and sometimes hourly, and have been for years. The only thing you can do to protect them from themselves is to monitor them constantly, and keep them away from boys, which will be enough of a challenge by itself. I wish you good luck.