Flake,
Might not be what you're looking to hear, but as a guy with two kids that I
wish I could take up together in my -6, I'd just never consider it.
Legal/FAR issues aside (though they really do need to be considered), it might end up more trouble, and a lot less fun than you'd expect, even if you could pull off the two-seat-belt thing. My guys are 8 and 9 and thus bigger than your crew, but it still seems your kiddos would be spilling into your lap, or in the way of the trim or flap handle (if you're manual) or just plain squished. Then I imagine having to fly while fighting
four hands in the "hey Dad, what does this thing do?" rodeo, or the "this seat belt is uncomfortable, I'm taking it off" game.
In tight quarters, the "DAD, he's (she's) touching me" brawl might continue right through the approach and landing...bad juju, IMHO!
As much as I'd like to take them both to lunch or fishin', etc, and as much as my knucklehead 8 yr old
wants to ride in the baggage compartment
, I just figure to share flying the RV with them one at a time and use it as solo time with Dad (maybe I'll rent sometime...perish the thought...and take them both).
All intended in good spirits...Dad to Dad (I'm sure there's lots of Dad's out here that understand the desire you're expressing!)
Cheers,
Bob Mills
"Rocket Six"
N600SS
Reno-Stead (4SD)