Batters, you are right that a child who hits back is likely to get in as much trouble at school as the one who starts it - in fact often the retaliator is the only one who gets into trouble, as the start of an incident goes unnoticed.
But...but..."telling" becomes less and less effective as they get older. A lot of playground supervisors will say something like "don't be such a baby" or "go and play over there instead" and the hitter gets away with it. (And you can't blame them for that because they don't know if something was said or done to the hitter first...it's like the UN sometimes!)
(Example - DS2, aged 9, in the cloakroom at hometime yesterday - NB, this is just his version - new girl (aged 10) hit him on head with book. He hit her on head with bag. She hit him with book again - he hit her with bag again - she hit him with book again - he hit her with bag again - she kicked him in shin - quite hard - end of fight! He insists he hadn't said or done anything to her first but who knows. I said he can tell their teacher today if he wants...)
Children can be such devious little sods and some of them get very good at starting things and then playing the innocent, especially if they have been doing it for years without being either hit back or punished.
I agree that SofiaAmes's childminder should be being much more reactive, if not proactive - watching the other child like a hawk, preventing him from hurting before he starts or dealing as severely as she can if he does start, for his own good as much as anybody else's - it might be "just a stage" (and the imminent new baby might have something to do with it, I suppose) but the longer it goes on the more the behaviour is reinforced. Does she have a naughty corner or stop him doing things he enjoys after he has hit/thrown/bitten? How does she deal with it?
SofiaAmes, I do understand your not wanting to teach him to hit back, he is obviously a gentle child, but it's a rough old world out there and they can't always (or ever, in some cases) stand up for themselves verbally. He is too little to try to avoid trouble (which is another strategy I've tried as mine get older - "just stay away from him" - doesn't help if the person concerned comes looking for them.)
Sorry, I'm rambling, don't suppose this helps much! Good luck anyway...