You cannot use any sort of leveler with a booster, you can't use a leveler on the sides of a seat, and you cannot ever use a wooden board as a leveler. You can use towels or pool noodles in the seat bight (crease)to adjust the angle of a rear-facing seat. They can't be used with boosters, with forward-facing seats, or to correct a loose installation.
Rae, how tall is your 7 year old? You mention that he's "big." If he's really tall for his age, he might not be too far off from being able to ride without a booster. The average height when kids will fit the adult seat belt in most vehicles is 4'9" tall. Of course most 8 year olds aren't that tall yet, but some are.
You might want to consider a bubble bum for the oldest child. It's very narrow and may fit between the contours of the seat better.
As chickabiddy said, you cannot place things under car seats to level them out. The only exception is to place pool noodles or a rolled towel in the bight of the seat to recline a rear-facing seat. And even then not all rear-facing seats allow it; you have to check the manual.
~*~ Full time mom ~*~ Hobby Sheep Farmer ~*~ Child Passenger Safety Technician ~*~
I think I just found that on car-seat.org, chickabiddy. (But WHY isn't it more visible or easier to search for?! It took me literally 3 hours to find a thread that happened to mention it. ARGH)
(and crap. Now I have to drive the van come December. blech. At least we know they fit, even if they are uncomfortable, in case we need to use the Prius in a pinch.)
ETA: brigala, he's not *that* tall, and I don't suspect he will be anytime in the next year. He's almost 4'3" (I think. I've been quite lax in measuring my poor kids, lol).
I'll check into the bubble bum, but now DH is saying we may just get a new car. :sigh: Allegedly starting in 2010, the back seat of the Prius is a flat bench, rather than contoured.
homebirthing, homeschooling, natural parenting, attached mama to DS1(3/05) DS2(3/08) DD (12/12)