Unacceptable on many levels. The first thing I would do is write down everything that you remember. Details get lost sometimes....expecially when you are very upset. List names and times to the best of your recollection.
Second, contact the corporate office...immediately and often (every 24 hours if you don't receive a response)...though they'll likely jump all over it.
Third, do contact your state daycare licensing agency. But also be prepared to hear that they can't help you. Not every state licenses these types of facilities...not all are considered daycare since the parents are in the buidling.
Three caretakers is plenty for 10-15 kids, but they should be dividing duties by age groups so they are each responsible for a certain number of children.
At a minimum, I'd like to see their procedure for dealing with an instance like this. The manager should have been contacted the minute they couldn't locate your child. In my company, there are lock-down procedures that go into place if a child goes missing from the play area. Kids get into the darndest places, but there should be back-up procedures to mitigate the circumstance.