I feel for you! I've had students in the past whose parents are insistent that their child is the next piano prodigy, or simply think that piano lessons need to be part of everyone's childhood--whether the child wants lessons or not. I've had a few things work with these kids:
Incentives (as others have mentioned): students earn a sticker for each page passed in each book. We keep track of these stickers in a folder, or even in their theory book. Once a student has earned 25 stickers, he/she gets to pick a prize from the 'prize pouch' (prizes are very inexpensive and small: piece of candy, eraser, stickers, trinkets that you'd find in the party favor aisle at the dollar store).
Keyboard time: I bring my keyboard out for lessons with some students as a motivator. Students like to change the timbre (e.g., vibraphone, strings, I even have a 'laser beam' setting!), as well as add rhythm accompaniment to what they consider 'boring exercises' from their books. I also like to use the keyboard for young students with poor fine motor skills (the keys are easier to push!)
Duets: The piano exercises/pieces are much more interesting to the students when I play along with them.
Piano Games: For the students who loathe lessons, we step away and play many piano-related games (at least they're getting their theory in, right?!). One favorite of all my students is the m & m game: two cups are set out (one for them, one for me), and big ol' bag of m & ms (or skittles, or any tiny candy). We go through note reading and/or theory flash cards. If the card is played or answered correctly, an m & m is earned. Answered incorrectly, and I get the m & m. Students don't want to give me their candy, so they are highly motivated to concentrate and practice prior to the lesson!
Sorry, I went on at length about this. Hope any of these suggestions helps you out. I feel your pain!
