For both improved safety (seat belt/shoulder harness placement on the child) and visibility you should use an approved booster seat, not a pillow for your child. Obviously the preferred choice would be to use the car seat combined with a headset with smaller ear cups, as it will provide far more protection for your child. The data on the value of the use of approved car seats for young children is so overwhelming that it is literally "criminal" not to use them in automobiles. Unlike cars, general aviation aircraft have little or no crash protection other than sets, seat belts/shoulder harness.
As a dad of 8 I recognize what a pain in the neck car seats are!
