Thanks again to the group for all the insights. This really helped.

Child headsets- hadn't even thought of that! I just found out that my
FBO has none but apparantly had just ordered a couple. I really doubt
the 3 yo is going to keep anything like that on his head, though.

As for the seating arrangement, I had presumed that it would be a bad
idea to have one of the kids with a yoke in front of him but after
considering this advice I see that that was hasty. The 5 yo *should* be
old enough to be able to keep his hands off it if told.

I know the 3 yo has a problem with carsickness- yeah, he goes in back. :-)