We have often said that leaving a dog and a child younger than 6 alone together is not a good idea. That’s true no matter how gentle the dog and no matter how well behaved the child. A preschooler may not realize she’s hurting the dog if, say, she wants to press her pet’s eyes to see if they’re squidgy. Or she may want to play with the toys of a possessive dog and may misinterpret the animal’s displeasure. A new study only strengthens the case for not leaving small children and dogs to their own devices.