I have a DS90 for the kids as well, it is the 2 stroke model and has been great for the last 4 years. it's an '03 or '04, can't remember...I've only had to replace the battery on it.
I have a DS90 for the kids, not a bad little quad BUT, take a look at who makes them. (The 07's are made by Herr Chi or something like that) So basically I bought a chinese knockoff and paid $1500 extra for some upgraded plastic and some stickers.....
I originally was looking at the DS70....BUT, when the kids get bigger...you still have a 70cc.
If you go with the 90cc, you can limit the throttle to be the same as the 70cc...but when they grow, you have the 90cc that can still keep up.
The 2 machines are exactly the same size, just different motor.
Talk to your BRP Dealer about advantages/disadvantages with the 70 vs 90.
Plus, don't forget you will get better service with the Can-Am compared to the cheaper models...