#1babysitter
New member
While I love the Graco Nautilus, I really think it would be too wide to comfortably fit in the center back between two older kids. It is quite a bit wider than the Marathon.
Another poster suggested the Safety First Complete Air for the baby; you could actually buy that now and use it ff for your 6 year old as it's quite a bit narrower than the Nautilus. Then when the baby outgrows the infant seat (either by reaching the weight limit or when his head comes to within 1" of the top of the shell; the stated height limit is indeed arbitrary) you could reassess her maturity with regard to booster readiness.
The Complete Air should give you about the same length of harness use as the Nautilus. My super long torsoed grandson had totally outgrown the Marathon by the time he was 4, but could still fit (barely) in the Complete Air earlier this year (he is now 6 and recently outgrew his Nautilus harness by height).
If the 6 year old has outgrown the Marathon then using the Complete Air for her will not be an option since they both have 17 inch top harness slots as do the Radians. The Nautilus and Frontier are the only options. The Frontier is overkill in my mind since her kids are tiny and the 6 year probably won't be using the harness for much longer. Not to mention the Frontier is a lot wider. The benefit of the Nautilus is that it turns into a HBB and LBB. Another option is to put the 18 month old in the Complete Air in the 3rd row middle seat and your 6 year old in a Nautilus or Frontier 80 in the captains chair. The Frontier 80 has the same top slots as the Nautilus but converts to the highest HBB out there. You'd just have to deal with climbing in the back to buckle in the 18 month old. If it was me I'd put the 6 year old in a Nautilus back there.