I didn't mean to imply you're not a good parent or that your children are not responsible. The only apps like what you seek don't appear to work. I certainly would not rely on them. So, what do you do? I have grown kids and young grandkids. I worry too. But having a talk about consequences is about all I can do. Then I must trust them to do the right things. But I still worry. You're a good parent and you're doing the right things, but those apps won't help. Good luck!
AT&T has a free app that disables texting when you are traveling at 15 mph or greater. The reviews aren't great, though. Some people claim it only works with SMS and messages take a while to arrive to arrive after they stop driving. If you have AT&T, you might want to check it out. If may have been improved with an update.