Boa c. constrictor will definitely outgrow your 6'-7' range. While I have not kept them personally, so I can't really say what their temperment is like, I have read enough to know that they can get quite large. Boa c. imperator will most likely be what you are looking for between the two. There are a lot of different Bci, the most common is the Colombian redtail aka common redtail (as seen in pet stores). In my experience, the Colombians are generally calm in nature overall (I have 5 adults and 3 juveniles). If you go with the Colombian redtails, I'd recommend a male as they generally stay in the 6'-7' range, whereas females may (I stress the 'may' part) get up to 8'-9' range. FWIW, of my 3 adult females, two are four years old and 7' and the oldest is 6' and ten years old (give or take). I expect that both of my four year old females will top out around 8'. Both of my males are 7', one is 12 years old and the other is eight (give or take).
There are also Central American Bci. The few Central American Bci I've seen are smaller than the Colombians but can also be nippier initially. In my own experience, gentle and consistent handling will go a long way towards calming them down. Sonoran Desert boas (yet another Bci) are also smaller than the Colombians and seem to have the same gentle nature, at least the few I've encountered have. The specimens I've seen are overall darker than the Colombians but still very attractive snakes. As already mentioned by the other posters, Hog Islands are also very nice Bci that stay within the size range you are looking for and can be quite docile.
There are also a couple of other smaller boas that I'm unfamiliar with - Crawl Cay boas I've heard mentioned are smaller boas but I'll be the first to admit I know nothing about them. Also the Corn Island boas are smaller (there's a post above by Gus of Rio Bravo on these). There are also Solomon Island ground boas (a personal favorite of mine - three in my collection) and Viper boas (also great little snakes). Both are subspecies of Candoia and both remain relatively small - males around 3' and females around 4'. Captive bred is the way to go with these little beauties and you should have a docile small boa. Rosy boas are also smaller though I've heard mixed reviews about their temperment. Sand boas are a great choice for a small boa, usually maxing out around 3' and easily housed in a ten gallon tank their whole lives. Rubber boas also stay relatively small but aren't as attractive, in my opinion anyway, as some of the others.
Alot of the temperment concerns will vary between the individual snakes, obviously, but overall Bci are calmer and more easily handled as well as some of the other small boas I mentioned. As for interaction with your children, I would suggest starting with a neonate or juvenile boa of whichever species you decide on. The small boa will be less intimidating to small children, though on the same token they can be nippy until they are used to being handled. However the bite from a neonate is nothing compare to a bite from a larger adult. I have 22 snakes in my collection currently and there are some animals I will take out around children and some that I absolutely will not. It is just as important to teach the kids how to behave around the snake as it is to teach the snake that handling is a nice thing.
Whatever species you decide on, best of luck. They're all great! 
Raven
