I think the problem is two-fold. First, the airlines need to do a much better job of informing the public that if you are a "person of size" that you are required to purchase two seats. If a person has never flown before, they'd have no way of knowing that this is required. Not knowing can cause financial hardship (having to come up with several hundrend dollars in order to board the plane) and of course extreme embarasment (even if handled descretly by the airline rep).
The second issue is people of size who KNOW of this rule and are quite aware of their size, and don't follow the rules. Either the airline (wrongly) doesn't call them on it, or the airline does ask the customer to purchase an extra seat and the customer raises holy heck about it.
As for kids being split from their parents, the main reason this happens is due to overbooking by the airlines (which is legal, btw).
Lets say a plane has 100 seats. The airline oversells by 20, betting that at least 20 people will cancel at the last minute or just no-show on the flight. The first 100 people that book get seat assignments, the last 20 that book do not. The airline is right, and 20 people don't show up for the flight, so all 20 oversold passengers get a seat, right?
Well, the 20 people who don't show, open up seats all over the plane, in one's and two's and three's. The airline does it's best to keep traveling parties together, but they can't always and something has to give. The flight attendants will ask for people to switch, but often no one volunteers (I never could figure that out, sure some people have good reasons for not moving...but someone on the flight must be able to!). The result is that, sometimes families have to split up. The airline and flight attendants aren't trying to be mean, it's just a by-product of overselling. And before you all start complaining about overselling, it's just this practice that allows the airlines to sell tickets for $150 round-trip instead of $350 round-trip.
Moral of the story, if you MUST sit together, make sure to get seat assignments when you book your tickets (on airlines that have advance seating). Also, keep checking your reservation once a week or so, equipment or schedule changes can cancel your seats, you may have to re-select your assignments.