Looking for new seats!!!

All advice welcome here ladies!!!
Iam sad to say that my sister does not use the proper seats for her 2 boys It scares me to death to think of them not restrained properley in the van......So with my nephews BDAY commin up I have decide to get them new seats!!!
Becasue I dont have alot of money and Iam getting 2 seats Iam looking for the safest seat for the best price!

Ok so the boys are 5 and just about 7!! However the oldest boy has autism and does not like a seat at all!!! He seems to be able to get out of his booster that he is in now so Iam thinking a 5 point might help this (can a 7yr old be in a 5 point??) The boys are tiny in weight the 5yr old is about 35 pounds?? and the 7yr old is 45?? They are only about average in height also!! If I need to find out specific stats just let me know!!
My sister drives a 2002 Pontiac Montana!!! Oh and Iam in east coast canada girls!!!

So what are you thinking would be good seats??

The 5 year old legally needs to still be in a 5 pt harness. A Radian would be a good choice for him.

If the 7 year old is short enough, you could do a Radian for him as well, but he's probably right at the top slots... so not too practical of a purchase.

The Radian has the tallest top slots of any seat in Canada. If the 7 yr old doesn't fit that, then his only option in Canada is a booster. The Britax Parkway and the Graco Turbobooster are the two Canadian options that come to mind.

The Radian is at Canadian Tire for $199 (48lbs stickered limits)and also at Babyproofingplus.com for $229 (65lbs stickered limits - the website has the incorrect info posted... all the babyproofingplus ones come with the 65lbs limit).

Both booster seats listed are approximately $120 at Toys R Us.

Edited to add: If you can find out height of both boys as well as torso height, I can give you a better idea of how the Radian would fit them.
Torso height... have them sit on the floor with their bum up against a wall. Make a mark at the top of their shoulders. Measure from the floor to the mark... this is torso height.

Here's my 5.5 yr old in his Radian. He is currently on the top slots with about 1.5 - 2" of torso growing room. I expect him to last another year or two in this seat. He is 35lbs, 44in tall and has a 16" torso...
http://farm1.static.flickr.com/125/413192536_e6bcc9cb12.jpg?v=0

I will get their torso hieghts in the next day or so thanks for the advice

FYI
I was in a Canadian Tire in Nova Scotia yesterday and the Radian was priced at 159.99 until Sept 14th.