I don't drive the children, but my initial thoughts are that I would harness as long as I could for DCKs. I mean, my 6yo is still in a 5pt harness. My 3yo turned forward facing just a few months ago. I believe in harnessing kids as long as possible and even if a parent had their child in a booster in their car, I might feel compelled to keep them harnessed longer for my own peace of mind while driving.