There was a local team in my area that used the gun pretty cleverly I think for a 10 year old ball team. They clocked all their pitchers after about 1 month of practice and offered a prize to kid who was able to increase their velocity by the highest percentage by the end of the season. None of the kids knew there was going to be a prize given when they were first getting clocked and they all threw at their fastest possible speed for two stikes in a row (young ego's). The coach told me all those kid's increased over 20% during the season, all on their own accord (prize driven, of course) and he went on to use the results as a "soap-box" speech to his kids on how much they improved just because they worked on improving themselves in other than the normal practice times). I thought it was clever use of the gun at that age.
I personally think the Gun is good (just bought one but have not used it with my son yet) to use as a measuring stick for both velocity and change of speed pitches (the latter is why I bought mine for, we are trying to develop a good change-up this year (11 year old)). I think it could be used to show improvement when working on mechanics at this young age even if you were just throwing fastball's but again Adult supervision should always be present to ensure that the boy's don't abuse the purpose of the gun in these instenses. Just my thoughts