Transitioning a child to a new way of eating can be difficult. Especially the older they get.
There is a great blog post I recently read with some great tips to help the transition.
I do want to add that I talk to my kids - even my 3yr old son, and explain WHY we are doing something diet wise (though I use simpler terms for him).
And I always bring it back to God - our bodies are a temple for the Holy Spirit - and we are to be good stewards of that temple. While I believe God can use anyone (even an invalid) to do His will, if He gave us an able body to do Hill will, should we not do what we are able to keep it healthy enough for us to be able to go out and do His will?