I would never consider riding an upright bicycle without a helmet. Having said that, while riding a trike or tadpole I really do not see the need to wear a helmet riding around our neighborhood or on a paved trail. Having said that there are probably some trails that have hills that under the right conditions a helmet might be necessary to avoid a head injury.
We are getting a pair of Hase Kettwiesel and my Son is autistic and really dislikes anything on his head and it will really detract from his enjoyment. I will be the Captain and very conscious of his safety. By the same token I know he will really enjoy going fast as he is fearless on roller coasters.
Riding on the road I will make him wear a helmet.
Appreciate any comments or insight.