In season 1 christen says that Boyd is 2 years old. In the following season she says that he is five years old.
^^^ This is common practice, especially in Comedies, to age a kid from baby to 5 while no one else ages. the problem in this case is that Kristin is still 20 years old which means when she had Boyd she was only...well, do the math! (Anyway it's not a goof).