You can always buy fewer meals on the dining plan than you have people on the dining plan.
You should see if you can look at the meals first, because the WDW kids' meals are very small and children would have to have extremely small appetites to be satisfied with splitting one. Also, they can't split buffets, all you can eat restaurant meals, or CRT/Akershus. Everyone gets charged for those.
If they split all their TS meals you'd also have leftover kids TS credits that you would have to use somewhere unless you are okay with letting them go unused.