i have a co-pilot on one bike which my (petite) 5yr old daugher loves, since she can see where she is going, play with my disney princess bell (the shame) and pull the brakes on when I’m least expecting it.
I have a Humax on the back of my 29er which my 2yr old son loves, he’s got cerbral palsy so the co-pilot is a bit of a non-starter since he’s not got the necessary stability and can’t use one arm yet. that said he yesterday demonstrated the abilty to pinch monster munch from the pocket of my cycling jersey with his less affected hand
I used to find my daughter would get bored in the Humax and fall asleep, no such issues when on the front.