I was a banquet server for years and frequently worked large buffet events where extended families were frequent guests. Those big-tables tended to be the hardest work for me, even worse than for table service, and people tend to notoriously under-tip when they eat in large parties (which is why the restaurants started adding it automatically, of course.) From my experience, I would say that 10% is fine if there are six or fewer at the table, but for a larger party, 15% at minimum would seem to be appropriate.
Buffets cause guests to get up and get a new plate several times during a meal, which means that the server is normally clearing the equivalent of 3-4 times the number of places than he/she would for a table service meal. Add a large number of kids into the equation, and the cleanup after the party leaves can become nightmarish, and almost always requires the floor to be swept before the tables can be reseated.
The other reason for tipping as for table-service when you eat in a large group is that, even though buffet service is faster, large groups tend to linger. This means that the table will not turn over as many times per meal as it normally would, leaving the server with fewer tips. For example: if you have 6 4-tops in your station that would normally turn over 4X each meal, you can expect to collect tips from 24 parties in one shift, at approx 10% of each total. If you instead get a party of 16 and a party of 8, you will probably only turn the tables twice for that meal (because they socialize more and the setup takes time to arrange), thus seating 4 parties, and probably get less than 10% from each party. (Most restaurants do NOT pass all of an automatic gratuity on to servers; usually they get 50-75% of what is charged. I don't know if WDW gives 100% to servers, though; they very well might.)
As for the $1 "buffet tip", I got out of the serving business 15 years ago, but even then that was an outdated idea. The IRS taxes waiters on the assumption that they earn a certain minimum percentage on all the checks they serve, and if they don't actually make that much in tips, they have to pay taxes on that "income" anyway.