The backseat-doors of a police car have no interior handles and therefore cannot be opened except from outside; also, a State Trooper never goes on patrol without a partner.
There was no need to involve the children in finding Leo. When Leo got access to the phone at Howard Piggan's house, he could have simply called 911 and reported that the missing cat was being held in the house. The police would have tracked the phone number, picked him up and brought him right home.