aha so its not really the killing that's concerning you - but the body contact right? if i understand you alright?
i dont know where you are? what is the weather like?
are you guys able to go out a lot?
to me it sounds more of 4 children needing to find a better outlet for their physical 'madness'. i notice the moment spring comes i have to do more outdoor stuff or else dd starts 'acting' up. after being cooped up for winter. could this be happening to you?
so are your kids breaking down due to the punching match, lots of crying? i think you can guide your 2 year old on this but the others you can draw boundaries. however if your 5 year old is going thru what i call the 'angst' phase where they act 'hormonal' then that child may be the worst hitter. it is a stage they are going thru.
oooh i also see you are pregnant.

CONGRATULATIONS!!!!