Awesome topic!
Yeah, it's hard to draw the line but I think I've come up with three things that ought to be taken into consideration:
Ride System- Is the ride hardware designed to handle more of an extreme experience or one that's more mild?
Branding- Is the ride being marketed to the public as a crazy, white-knuckle thrill experience or is it being promoted as something along the lines of "the great family adventure?"
Public Reaction- Does the ride extract bloodcurdling screams of terror in every element in a train full of adults or do guests on this coaster appear slightly more calm?
Automatically disqualified if the ride A.) has inversions or B.) has a height requirement of over fifty inches.
If the first point ended up being true for two or more of the questions I posted, it's not a family coaster. I'm a bit cautious as to what I'll assign that label to though, as I believe lower intensity thrill coasters can still provide insane sensations. Arrow mine trains, FireChaser Express, and other similar rides I consider family coasters. However, some coasters with "big boy toy" ride systems that some might consider family I consider to be thrill coasters, such as SooperDooperLooper. Cheetah Hunt, by far the most controversial coaster in my top ten, I actually consider to be a thrill coaster because while it was marketed to more of a family demographic, that blitz system is still in use on Maverick and iSpeed and the screams that come off of it sound exactly like the ones on Montu.
I don't mean to be "that guy" and derail this discussion, but I've actually seen people on Facebook calling Lightning Rod a family coaster! There's been talk going around that the reason it has a low height restriction and no inversions was because the park wanted a family-friendly experience out of it. Of course I think that "accessible by" and "designed for" are two different things, but to me there's no way you could classify something that feels that fast and dishes out a ride experience that violent as a family oriented experience, even if kids are able to ride it with their rents. Is there maybe any other justification for classifying RMC's most violent ride as a family coaster? Other than "the whole family can stare at it closed together?"