auto policies with different limits

You knew this day would come. Your teenage daughter has bought a car and needs insurance.  With your encouragement she shops around, buys a policy and puts the car on the road.  All's well and good until that Saturday morning when you wanted to get coffee and her car is the last out.  No problem - you'll just take hers rather than move cars around the driveway.  On the way home you're distracted reaching for the coffee cup and bump the car in front of you.  Problem!  You have a $1M umbrella policy but it requires you carry $250,000 / $500,000 bodily injury limits and your daughter only bought $25,000 / $50,000.  Big Problem.  Here's the deal - as the driver of her car you are protected by her insurance limits not yours.  The personal auto policy form doesn't allow 'stacking' limits between residents of a household.  If the other driver sues you for bodily injury of $50,000 and wins your daughter's policy will pay $25,000 and you'll be on the hook for the balance.  Your own policy won't respond and neither will your umbrella.  Solution - either make sure your resident kids carry the same auto limits as you do or don't drive their car - ever!