its an interesting one, i am trying to get my boy interested in MMA or a martial arts of some form at the moment he is 4.5, main reasons are for disapline, respect, develope hand eye skills etc etc, however i dont think there is benefit of doing full contact combat sports at such a young age, even though i competed in many a judo comp from the age of 5 it is completely different to full contact MMA.
if there was some for of head protection and bigger gloves if its possible such as in amatuer boxing matches then this is a different story...