Don't think Hyman deserves to get mentioned here. He only gets knocked down because he's totally relentless battling for the puck, he'll go in recklessly even when he doesn't have good body position/balance, just to start initiating contact to throw the other guy off. It often works, he wins tonnes of puck battles that he has no business winning, simply by throwing himself into them. He's arguably #1 on the team in terms of ability to win battles, he just puts himself in compromising situations where he's prone to getting knocked off his feet because he hustles so hard, and battles like crazy whenever the puck is remotely close to him.
Marner is different, he DOES get knocked down because he's skinnier than the opposition, he gets straight up outmuscled constantly. Again, though, I'm not worried. It's a very common issue for skilled, skinny youngsters, and almost all of them make big strides in that area as they age. His high skill game means he doesn't need a tonne of power, he just needs to get to mediocre NHL strength for it to be a complete non-issue, and I'm very confident he'll get there by his ~mid 20s.