"Every pet" is singular and "their" is plural. They must agree. I think the simplest way to correct this would be to change "their" to "its" rather than "his/her" since the pet is not actually a person. (My daughter would be offended by this since she takes her Webkinz pets very seriously.) I think the rule with animals is that if the animal is named, you should use "his/her," but if there is no name for the animal, "its" is appropriate. I think that stuffed animals should always be referred to as "its."
