What would be a good seat for the occasional extra kid? My son is 4 years and in a GN. I'd like something to put in the 3rd row of our Sienna for his friend(s), carpooling, etc. Is a harness necessary? Still safer? Or will a booster be as safe? Most of his friends are 4-5 years old, with the occasional 3 year old son of another friend of mine (she has him FF in a Britax Boulevard in her own car). If a harnessed seat, strap adjustability must be easy, obviously, for a good fit on each of them. Suggestions? Price is not a major concern for the right seat.