If dogs and cats had thumbs, having them as pets would be different in the following ways:
1) Dogs and cats could manipulate objects with their thumbs, allowing them to open doors or cabinets, turn on electronics, or play with toys in a more human-like manner.
2) Their ability to grasp and hold things would change how they are trained and entertained since they could do more activities that require fine motor skills.
3) People may treat dogs and cats more like family members if they seemed more human-like in their ability to use their paws, but they would still need training to ensure thumbs are not used to misbehave.