Hasn't been an issue for us yet, but when the time arises, I won't travel without my kids in separate seats, regardless of the cost. (And by the way, I am as cheap as they come, and don't make big $$ by any stretch!)
1. Adult comfort. We grow 'em big (Patrick is 18 mos. old and over 35 lbs - all in the shoulders). Victoria (3 in May) is almost 40 lbs. She once climbed on my chest and fell asleep while I was napping on the sofa. I woke up 30 minutes later thinking I was having a heart attack!
2. Kid's comfort. The more comfortable a child is, the less fussy he/she is going to be.
3. Safety. The odds of anything happening are minute. Nevertheless, in the event of some nasty turbulence or a hard landing, they'll be exactly they should be.
4. Sleep. Our carseats have wings for the headrests. Patrick nods off within a couple of miles and sleeps comfortably.
5. Familiarity. New experiences can scare kids. The comfort of their car seat plus the companionship of their favorite blankie goes a long way to relieve stress.
When I bring Patrick to visit my mom, he doesn't sleep well in the portable crib. However if I put his carseat in the crib in a reclined position, he sleeps until morning.
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"I will not be pushed, filed, stamped, indexed, briefed, debriefed or numbered. My life is my own."