When doing a tight three across, my first suggestion is always to use seat belts to install *all* seats b/c it can buy you an inch or two of space, which makes all the difference in the world.
My second on is to consider a Frontier if you can swing the space and/or money b/c dealing with a seat belt (doing it and/or undoing it and/or having the child crawl in/under it by leaving it done up) is a serious pain!
The external seat width of the Frontier (19") and your Graco SS1 (19") are both the same--though I'm not sure if that includes the width of the base which in my experience is wider. Obviously, otherwise almost all 7yos are more than ready to go into a booster seat, so there is no safety need, simply an ease of use and even convenience reason to think about trying that as an option. And, that would also come from my own experience of dealing w/ an occasional 3 across in my car for car pooling and it being a serious pain and taking a few minutes to buckle my son's friends in while pushing/pulling/poking my hands and face where it otherwise doesn't belong!
As for your question and the most practical and cost effective thing to do at this time, boosters.
The SKJP/Diono Monterey booster is 18" wide. Make sure you try (all/any seat) before you buy, the Monterey is known for having some incompatibility issues w/ the belt guides (it falls out and can be annoying). I like the SIP of the Monterey, but do not like the belt guides or that having a head rest is required, even thought it's a given that it's a good idea, required makes it more serious in my mind. All the booster measurements are found
here.
TurboBooster is 16" wide. And, of course I would put a 7yo in a high back booster, rather than a no back booster. And,
the Britax one is 17" wide.
Also, just b/c one is 16" vs 18" doesn't mean the 16" one will and 18" one will not fit, it's more complex than that. Sometimes the wider seat will fit b/c of the way the arm rests sit/fit.
Finally, I would look at how and where your seat belts buckle and place the booster in the place that is least likely to create an accidental unbuckle on the part of the booster seat beside a car seat. I have a friend that just moved her 3yo centre w/ boosters both outboard. He'd been there for a day or so before she realized her son had accidentally unbuckled the car seat and she'd been driving like that...I simply had her flip the belt stalk of the car seat backwards so it wasn't in the way AND put the prickly side of a velcro strip on the button so he'd feel if he was pushing the wrong button again.
Hope that helps!
tam
