Right wanttobehealthy, when someone shows you who they are believe them the first time!!
Learn from this. Next time show up at the house and wait outside while the kids get ready. The kids will know that you are always there for them and would get in the car with you if dad "couldn't find his car keys". I am sure you will never let this happen again.
So just learn that you can not trust him to do something like this as he is not responsible enough. I would get her the phone, charge it before hand and have her hide it in their underwear. He will take it from them if they new they had it. (more drama). Show her how to text so if she can't call she can text you that she is ok.
Try for the future to be proactive not reactive and they will be ok.
Keep the Faith!!!