I think this is the sort of thing that depends on the degree. My older son (2.5) usually is an incredible eater - he can eat 8oz of meat in one sitting, and is equally likely to eat 8oz or more of broccoli - but occasionally (maybe 1x/week) doesn't want to eat much at dinner. As such, we don't worry much about these spells; either he really isn't hungry, or he has something else interfering (desire to play, etc.), but he's clearly eating enough overall (he's quite tall and of appropriate weight). So we simply require him to stay for the meal and do not permit dessert on these days if it is available. If he is hungry later, he may have his uneaten food - we save it in the fridge for the night, still on his plate.
I'd also say that as the children get older, this should be more 'up to them' so long as the consequences are understood (they may have leftovers, for example, but no new food - for example, no PB&J sandwich or cheese or similar food they may prefer to dinner) and it's not too frequent. While a 2 year old might need some parental intervention to make sure she eats enough, a 4 to 7 year old should be competent at judging hunger.
On the other hand, if this is an issue most days, then more significant action may be required. With children old enough to discuss their reasons, do so - find out why they aren't eating with the family. Let them know that it's necessary to eat with you, if you decide it is - and if they have good reasons, adjust for those. They may not feel hungry at the time in which you serve the meal - if they have after-school snack at 4:30 then a 5:30 dinner time may be too early, for example. If they don't like the food you prepare, perhaps involve them in meal planning.
If they simply don't feel like eating as a family, and it is important to you as parents to do so, then step up the consequences - stick with logically related ones, where possible; for example, start by prohibiting eating after dinner, and then perhaps extend to prohibiting gaming or TV watching after dinner if they don't participate.