It's important. The times that it's critical are rare. So... add if's it's in the middle of the road you prefer to stop rather than run over it. If it's up-right it's proper to dodge dangerously rather than to hit it. The number of crawling kids in the middle of the road is quite small, but it's larger than the number of infants, so add in something that smoothly increases the probability of human as it's (estimated) weight approaches 90 pounds and decreases it as it exceeds 300 pounds. Or 400. So you have a flattened bell curve with a smooth top.
But really, all this fiddling is just to handle corner cases. Usually you just stop or avoid the thing on the road without wondering much what it is. Only if you can't do either of those do you need the fancy figuring, which is a pain, because that's when you need the fast decision, so you "corner case handler" need to be something simple.
Rule 1: If it's standing up, it's a human. Don't hit, even if you must take damage. (This yields several false positives, but too bad. We need a quick decision.)
Rule 2: Estimate it's weight. (Ouch! That looks like a slow process...so while you're doing it, slow and start dodging.) If it's above 25 pounds, avoid even if you must take damage. (Note that hitting something heavy at a fast speed will damage you no matter what.) Continue slowing and preparing to dodge. If it's following a ball, dodge even if you must take damage.
Sorry, time's up.
This isn't a perfect approach, but it's simple, and doable. The hard step is estimating weight. There is a problem with false positives. A paper mache statue would count as human. But it should handle all common cases. And there should also be a distinction between streets where the traffic is slow and rare and streets where the traffic is fast and common. Freeways are much less likely to have humans walking in the road.
Additionally, there should be a rule about not overdriving your reaction time, especially on slow streets, but nothing can stop a kid from running out right in front of you from between two parked cars. And nobody, neither automaton nor human, can reliably deal with that. Which is why that first rule about "upright" is made to yield a lot of false positives. If you have time, then you can refigure things and perhaps decide that "that's a paper mache statute", so you may start to dodge in a way that will damage yourself, and then refigure to avoid damaging yourself when you, more slowly, decide that such action isn't needed.