The dealerships will provide free loaner seats and coupons for new ones. Your order will be delivered before you arrive and picked up after you leave. You activate the toy from the front, making the lights blink on and off and the music play. There's an attached teething key and squeaker phone. Toddlers can press buttons to make the toy work themselves.
Older kids can be just as tough as a baby to keep happy along the way.
Klutz' whimsical glove-compartment guides might help. Of course that won't be enough to keep your gang going for the whole trip. These would also be great to tuck in children's backpacks before putting them on a flight by themselves. Or for that time they absolutely have to get out of the sun. Eat a candy necklace for me.