Sight reading seems best learned by practicing it at the lesson. I teach students to count a measure before beginning and then to try reading through a piece, and to keep going even when mistakes are made. This point is most easily taken when playing along with another person, who either plays the same thing as the student, or who plays an accompaniment to his part.
Christmas carols are usually a big hit among students, especially the young ones! They will motivate my students for the best reason: they want to play these pieces. The first few years, I encourage counting aloud for a few weeks before trying to sing. With very young students, a great thing is, they may not know these tunes yet. They don't mind counting aloud to learn the pieces. Singing the pieces is easily learned after students have counted for a few weeks.
Many Christmas books have accompaniments to go along with the student parts. I like to encourage older siblings or older students to learn to accompany the younger students. The first thing I teach about accompanying is to learn the LH part first. Adding the RH is usually much easier once the LH bass notes have been practiced. Here is a new brother and sister team learning to play together at lessons.