First of all, the most important thing is that your kids are in the safest seating possible for their age and size. My very tall and big almost seven year old is in the britax frontier, harnessed to 85lbs then a high back booster to 120. My four year old is still in the britax marathon to 65lbs, then will be able to go into the frontier because my oldest will be able to use a regular high back booster by that point or none. After a fatality accident, I don't take chances, no matter how much it costs. And their life is worth way more than anything you'll pay for a proper booster seat.
Tell your seven year old it's the law and it's safest for her, and she can maybe just use one of the cooler high back or no back boosters, get a britax frontier for the little one, will last as long as she needs one..$250 over 4 years isn't that bad! Check albeebaby.com or buy new in box on eBay. Most four year olds are not mature enough to sit properly or not mess with the seatbelt in a regular booster. Five point harness for as long as possible is safest! Safety ratings and reviews prove not all carseats are the same regardless of brands, it's not true that any old carseat at Walmart will be as good as another. The nationwide recommendation (may be different from laws) is 8 years old AND 4'9"! It's about bone density and makeup after a certain age, as well as how tall one must be to fit in the seat and belt properly.
Eta: grammarocks, I've been in three accidents and each time the carseat check was the first one they issued, separate from the car and a settlement. They know you need one right away!