I don't have any recommendations for you, but only an observation.
Imagine this possible situation:
You are told that the work for today is to pick up heavy stones on one side of the yard, and drag them over to the other side and build a pile. You, being a lover of the Lord and not afraid of work, gladly resolve to do your best to God's glory and have fun with it somehow. Then the next day comes. And the next day's instructions: move the pile back to where it was. Surely, you think, there is a valid reason for this, and you trust the one instructing you. So, again, you gladly seek to honor God and work hard at it, and you do well. The third day brings the same instructions, to move the pile back to the other side, and now frustration and a lack of motivation are inevitable.
It is human nature, and rightly so, to become unmotivated to do things that seem purposeless. In the short run, a simple order from a respected authority is plenty to inspire motivation, and should not need to be explained. But, when the hard work or assignment requires months or years of motivated work, a person will, every time, become unmotivated and a poor performer without a context for his work.
This story with the pile of rocks was actually based on actions by the Nazis in concentration camps. Reportedly, it drove nearly all the prisoners insane within days.
I guess what I'm getting at is this: they need to know not only that they should work as to the Lord, but
why the study is useful, and moreso,
why the hard work honors God, not just that it does.
An example: a young man must have a very strong working knowledge of the rules of grammar and sentence structure. Why? It is not because the effort itself necessarily honors God, but because God is honored by storing up necessary skills to recognize and reject devilish heresies, to defend God's word, to increase our knowledge of him, or to be able to work fruitfully later in life to provide for families and give to the church.
I struggled deeply with a lack of motivation throughout the latter half of college. And this was my key problem. Only, in my case, the purposefulness of the material was much harder to defend!
Thankfully, this is not the case for the basic education of children. You could even put the reasons more in their short-term: you need to learn how to read better because daddy would like you to help him in reading the scriptures out loud during devotions. Or something to that effect.
I'm not a parent, so no actual application
But I think the principle is relevant.