Part of the problem is the weight and speed - part of the problem is “solidity”.
If you are hit by a rock, there is no “give”. The entire momentum of the rock is imparted to the point of contact.
Get your kids to try this simple experiment - drop a golf ball on their head from 5 feet up, then a mushy brussel sprout. Repeat from 10 feet up. Get them to explain why the golf ball hurts more, even though the weight is not that much different. (Add a 1-inch soldi metal ball bearing if they claim weight is the issue…)
Basically, the entire momentum of the golf ball is transferred to to point of contact. A golf ball may be about an inch and a half across, but the contact point is about a quarter inch or less, depending on skull hardness. (That can be tested with a ball-peen hammer ) A sprout, OTOH, you do not absorb all the momentum in an instant. The part that first hits you stops. The parts around it keep going until they hit your head too. The center and top keep going - since the way down is blocked, they deflect outward - hence… splat!! Some of the terminal velocity is transformed into diagonal or horizontal velocity, not all the momentum is cranially absorbed.
Then there’s the question of terminal velocity. An object that falls accelerates until wind resistance balances the force of gravity. For humans, terminal velocity is about 100mph. Terminal velocity depends on weight and the cross-sectional area presented to the wind, so in some way it’s related to density.
If you want to experiment with terminal velocity, video dropping various objects from a tall apartment building, and count the frames to the bottom to get a drop time within 1/30 second. Leave before the police arrive.
Otoh, baseballs and hockey pucks travel at about 90mph to 100mph in the best moments of the game. I doubt brussel sprouts terminal velocity is faster. However, while you don’t want to be beaned by a pitch or slapshot, the damage is bad but not in-your-head-and-out-the-ther-side bad.
There’s the story about how Boeing was testing aircraft windshield resistance to bird strikes. They built an air cannon and would shoot chickens from the grocery store (the humane way to do it) at the windshields are aircraft speeds. The <insert favourite minority> Aircraft Company decided to do the same tests, but kept breaking windows no matter how slow they flung the chickens. They asked Boeing for advice, and a tech came over to watch and said “thaw the chickens first”.