Oh boy, here we go.
One: DO NOT BUY A USED CAR SEAT. DO NOT BUY A USED CAR SEAT. DO NOT BUY A USED CAR SEAT.
Am I being unclear? Should I rephrase?
I say this as someone who avoid toxic chemicals, as someone who makes her own cleaning solutions from vinegar, lavendar and tea tree oil, as someone who buys organic whenever possible, and as someone who is a vegetarian primarily because she cannot afford the cost of sustainable, organic meats.
This is a situation where you need to weigh OBVIOUS harm versus THEORETICAL harm.
1. Fact: there is no evidence, none whatsoever, that any child has EVER been harmed by the so-called toxins in a car seat, even kids who spend untold hours using them as baby seats (this is bad for other reasons, obviously, and should be avoided.)
2. Fact: there is clear and documented evidence from the harm done from not using car seats, using car seats with after-market products, or using used seats.
Car accidents are (with a little play here and there), the number one accidental killers of American children. Car seats are well designed, and do their job brilliantly.
The most important point to consider when purchasing a seat for your child is, does it fit your child, will it fit your car, will it fit your budget, will you use it correctly every single time you get in the car.
If you buy a so-called 'non-toxic car seat', but it doesn't fit well in your car and your child ends up getting killed in a survivable accident, that is an absolutely ridiculous tragedy.
There are seats on that 'non-toxic' list that I would never recommend for reasons MUCH more important than so-called toxicity, reasons that may mean actual death or severe injury.
OP, you do not need to buy new seats. Your children are not going to be harmed by riding in the Boulevards. The only reason I would purchase new seats is if they currently inappropriate for your children for a sound reason (For example, your 1 year old has outgrown the Boulevard rear facing. Then you absolutely buy a new seat that will keep him rear facing longer.)
If you are VERY concerned, one trip through the washing machine (the COVER, NEVER THE STRAPS) will take car of most of the 'chemicals' you are concerned about.