I've ridden with my daughter since she was 8. Now she is 20 and still rides with me. She has always had the knack of falling asleep while we are riding. In fact when I started riding with her I used to ride in dread and fear, constantly checking my mirror to see when her head would slowly tilt into view... I considered the belt thing but was disuaded by other riding dads who pointed out that in an accident the child may be crushed not only by the bike's weight but by your own. Far better to have the child thrown beyond the bike, far better of course to ride totaly and absolutely defensively...
For both of our safety I opted for a full back rest and top box, and a good intercom system. I cannot tell you the hours of pleaure, (and yes at times grating pain from off key songs), that I had listening to her invent stories, characters, give me full news reports about her life or the countryside we were passing through, or hearing her tell me that she loved me while riding in gale force winds and lashing rain.
All in all I think the experience of riding with your child, "DEFENSIVELY and CONCIENTIOUSLY" is a fantastic one if done safely and listening to their needs. We've done trips of 7 x 600 kms consecutive days together, and the trail of memories will always follow us...
...as for straps and harnesses I've never been fond of any form of capital punishment...