A few things you've written popped up for me here. So I'll try to take them one by one.
First, I think your care provider was likely very upset that the roughhousing was continuing, after she'd pointed it out to you before. Yes, she was empathetic at first, and because your child is continuing to do this, you are making this *her* problem. She made a boundary with you, this wasn't communicated well enough to your children, and now she's got another parent who is completely pissed off because their child was hurt. Whether your son "meant to" or not is really irrelevant, because he shouldn't have been rough-housing/wrestling in the first place. Period. If he was doing this, he was choosing not to follow the rules and expectations of the teacher. (I understand the other child might have also chosen to ignore the rules, and we don't know what sort of conversation went on with that parent, because teachers are not allowed to share that information.) You were notified beforehand, so I understand the teacher's upset and frustration. As a preschool teacher myself, I never, ever want to have to call parents for this reason. And you don't mention, but do you know the extent to which the other child was hurt? In any case, a situation like this has the potential to make the care provider look negligent, and we also don't want that either, as it hurts our reputation and this is our livelihood. (For what it's worth, when I started my business, I became an LLC and picked up a million-dollar insurance policy I pay out the nose for, because I don't want to get sued, lose my business or my home because of an incident such as this.)
You say that you've tried talking to them, but "they're boys". I have a boy, and I have boys in my preschool group. They do not hit each other because they know this is never acceptable, just as much as the girls do. I am careful to explain how their actions and words affect others, and we don't allow fighting talk/play at school or home. Please do not chalk this up to gender because while boys generally may tend to roughhouse more than girls, to permit this outside of a very supervised setting is doing them a great disservice. Your youngest one,especially, is a bit too young to be able to discern when this sort of rough play is and isn't okay. If it were me, I would be very clear that this needs to take a break for now, because people are getting hurt.
I'm also going to suggest you consider what other sorts of rough acting/violence/fighting the kids might be exposed to, if any. The cartoons your oldest might be able to comprehend don't come across as "pretend" to younger kids. Even in a lot of kid-directed media, there is a lot of fighting.
Has their father spoken with them about the matter? A firmly voiced conversation about why they need to stop the roughhousing, except at home and under agreed-upon conditions, can really help, especially if they see *he's* upset (because moms tend to get more upset than dads do).
Ultimately, though, this is a question of finding some new parenting techniques to stop this rough-housing. I would explain to the children what the consequences are for their being unsafe with their bodies, and stick with them. I would also do something that really gets their attention. Do they play video games or watch television? Perhaps removing these privileges with a simple explanation "There's no television and no video games for now because we need this time to practice being safe *all the time* with our bodies" will help them understand you mean business. If you don't present it as a punishment, but as a challenge the family needs to work through to restore a sense of rightness and balance you once had, you are more likely to get the kids on board. Give them goals, too. They have to make it for 5 days in a row with no rough-housing outside those sanctioned times, and then those favorite items might return. When the kids start the rough-housing again, start the "we need to work on this, no tv/video games" again. Incidentally, families who cut back on tv/video games often find that their kids get along better.
And equally important, give them alternatives for rough play. Do they need more time outside, to run, throw balls and jump around? Do they need things to work their muscles on rainy days, like playdough to pound or indoor obstacle courses?
Rough-housing is a tough one, because kids do need rough and tumble play, and they also need to have some really clear boundaries. At young ages, this can be a bit blurred, but we have to make it clear that A. it's an activity which requires mutual consent and B. you have to check with mom or dad first, EVERYTIME, before beginning to wrestle.