The only head injuries my two have had happened when they were wearing normal helmets, and full faces would have prevented them.
Son was pootling along and experimented with twittering the bars, went over, and landed on face. Suspected broken jaw (turned out ok at walk in) and killed the nerve in a tooth. About 3 or 4.
Daughter went otb at the pump track and landed on her nose. Was 5 or 6. Both blood everywhere.
IMO , they don’t have the same reflex s to protect their face that you develop a bit older. So if their necks strong enough, full face is a good idea.
Both now have one and always choose to use it at pump tracks, BMX, or mountain biking. They love them, don’t seem bothered by the heat ( but you can see the sweat), and have started racing bmx, 7 & 8. One is a 661 reset, the other a Lazer phoenix plus. Both adult Xs in sales.
They use the other helmets riding to school, they can hear better and that’s more important then.
The other priority would be small gloves. Long sleeves and legs seem ok so far for the limbs.