P.M.
I had several thoughts as I read your request. Feel free to adopt anything that fits, and toss anything that doesn't.
I'm not sure how old your kids are, but if any of them are mid-elementary or older, teach them what to do if they don't get picked up. Maybe ask the school office to call home, or arrange with a neighbor to be your back-up transportation (and offer the same service in return), or teach the kids a safe route home from school, and be sure they know what strangers are okay to talk to (teachers, police, neighbors they know, local merchants, etc.). Sooner or later, every child will have to deal with an unexpected situation, so give them some basic skills so they'll be more resilient.
As far as your ex criticizing you or step-dad, just smile sincerely and say, "Thanks for your concern. I'm sure you heard a version of the story that worries you. I'm glad you care. I'll be happy to tell you the adult side of the story if you want to hear it." Don't act defensive or make excuses, don't tell him he shouldn't worry or be mad, don't argue back, just don't put any energy into his criticism. The change in your response will probably surprise your ex and he'll lose some steam.
If you want to talk to your husband about your worries, look into the techniques taught in the process called Non-Violent Communication. You can google this for the basic process, tips and examples, and sources of books, videos and classes. It teaches you how to identify and communicate your needs and feelings clearly and respectfully, and listen to your spouse with clarity and respect. It's very useful.