The most important things to keep track of are these:
a. Is the child growing at a normal rate, and within a normal weigh range?
b. Are the foods they eat "real" foods, preferably fresh, and do they eat a variety of foods?
If these answers are both affirmative, then you have little to worry about. My grandson (now 5.5 years) has gone through long, long stretches where his metabolism apparently slows down, and he has very little appetite. Yet he is strong and always has energy to play, is mentally alert and positive, and grows at a normal rate. His weight has always been a bit on the skinny side, and if he gets a tummy bug, it's a bit alarming to be able to see the loss of a couple of pounds on his ribs and legs, but he always bounces back within a month or two.
Different people have different metabolisms, and some have remarkably efficient metabolisms that simply require less fuel. This is probably a good thing health-wise, as long as the fuel that goes into those systems are high-quality foods. Trying to get a naturally skinny kid to plump up is not good for their long-range health – once a fat cell is created, it's for keeps, and by middle age, even those skinny types tend to start packing on fat in unfortunate places.
If your concerns are serious, as in, your children are not growing or gaining weight normally, or they are dragging around listlessly, this is a concern to take to your pediatrician. But kids stomachs are about as big as their fists, and what seems like a healthy portion to an adult may simply be more than they can comfortably contain.