This is a very common thing among parents to tell their children not to eat sweets or other dessert foods before a meal as it will 'spoil their appetite', in the sense that they're not going to be able to eat any of their main meal, even from just one sweet.
I've personally been fairly puzzled about this claim*. My understanding is that one or two sweets (or dessert foods in general) don't nearly fill a child up enough to count for spoiling your appetite, at least as much as parents would want them to think, and there isn't much word about how that's the case. And how much would even have to be eaten for it to count as spoiling it?
Is it possible to not eat as much from a meal even from just one sweet?
*But then again, I am fairly young. Hur hur.