Asked tonight at the Soccer parents meeting if we should require the kids to wear them.
Here's the 1st part: They won't wear them unless they really fit well.
There are several papers on the use of mouthguards - but I can't find anything that is specific to this question: What is the incidence of dental injuries in soccer (Football in non-US papers) players - and is that number significantly reduced in players who wear mouthguards?