You should first start them on their knees, and do soft toss to their right and left, teaching them to use their non-glove hand to break their fall. ?I have done this many times with my HS and Travel teams, in the grass, as a warmup drill.
I also purchased a "Slide Rite" mat and have them work on diving that way. ?
Until they lose the fear of hitting the ground, it will be a difficult problem.
You may not be able to do this, but a couple years back I had an outfield that let everything hit the ground. ?I took half a practice, one day and I went out with the outfield and actually took fly balls with them. ?When they saw their 35 year old coach diving for balls, it then became a game, where everyone tried to outdo the coach. ?Man I paid the price for it the next couple days, but my outfield ended up making some outstanding diving plays that year. ?Not sure but I may have broke a couple ribs. ? LOL