Ditto Wendy. I don't own a car so the RSTV is (literally) a lifesaver. It's just so much easier to have kiddo walk to the carshare vehicle, etc. while wearing the vest.
Kiddo has browsed their website with me & I've always been in the habit of having her read through the manual with me. Even though she doesn't actually read the manual, I show her the pictures & we find all the parts to know their names & how they work. She has been using the RSTV now for almost 3 yrs & no longer needs any help to buckle herself in (although I often do it for her simply because other drivers don't like to wait so long).
I have found the sizing a bit inadequate. There is a little overlapping size range for kids who are just about out to outgrow the small yet not quite enough. My daughter grew too tall for the small, but hasn't quite reached the large weight minimum. After talking to customer service, I made the parental judgment to use the large. So, I recommend buying the small for your younger child 1st & testing it out on your older child before deciding. Your older child might do better with a booster. Or maybe I just have a weird kid