I had a Seat Jack on a 1998 Formula Z 670. It did the job with my 10 - 12 year old daughter. With younger kids it will give you more room as the driver, since it sits up over the top of the storage compartment. It was nice because it had small handles on each side for them to hang on to, and you could still get to the storage compartment. As a bonus, I could remove it in less than a minute when going riding alone. The one negative thing the kids had with it was that their feet didn't have anyplace to go - they kind of flopped around because they couldn't reach or stay on the footboards very well.
I also had a 2-up seat that I used on that sled, and the kids actually preferred it to the seat jack. They felt more "secure" on the 2-up seat since they could get their feet down better on the footboards, even though they had to hang onto me instead of handles.
When riding 2-up on a regular sled, there are compromises that need to be made - but it's worth it to have the entire family involved.
Good luck.