So put a helmet on them when they are walking down the road. Maybe some body armour too.
Your distinctions are totally arbitrary. You decide that you don't need helmets in the car because you are 'only' doing 80mph (40mph on what planet???), and you don't need them to cross the road, even though lots of kids get hit by cars crossing the road, but you do need them to ride a bike.
Is there any evidence for this conclusion? Not a shred.
Cycle helmets are an entirely arbitrary, given that we are more likely to suffer traumatic head injuries in cars, and don't wear them there (and forget this nonsense about driving carefully or whatever other rationalisations you have - you CANNOT ensure your child doesn't suffer fatal head injury in a car, and your only justification is that it's not as dangerous as F1, so well you accept that actually it's ok to not wear a helmet, as long as the risk isn't too high, but you feel sure that the risk is too high when riding a bike, even though you don't actually have any evidence to support that, and are incapable of making a rational choice between saying 'the risk of head injury in a car is 1 in 10 million, I will accept that, but I won't accept a risk of 1 in 5 million on a bike, and, er actually the risk is 1 in 4 million on foot, but ah well, nobody wears a helmet on foot, so why should your children.' You don't have those numbers so you don't know, you actually accept a nonzero risk that your child will suffer serious or fatal injury when you go in the car, a risk you refuse to reduce by wearing a helmet because you feel it isn't large enough, but you then go on to insist that you war a helmet in a bike, even though you don't know what the risk is there and don't know what measure of protection, if any, they actually offer.) And it's pointless analogising with seatbelts, the correct analogy with a seatbelt is another seatbelt, but bikes don't have them, but they also don't go at 100mph. Just because a seatbelt is essential when in a car, does not mean that an arbitrary other item of safety equipment that works in a completely different way, viz. a helmet, is essential when riding a bike.