I think that there is a risk that this can mess with future eating habits, although perhaps not exactly the way you expect. By dictating how much she eats, you are circumventing her ability to learn how much food she needs to be full. This is the case whether you state how much food must be eaten before she earns dessert, or you bribe her with sweets during the meal (which is really just a smaller bribe to earn a smaller treat).
Useful reading on this subject is Satter's Division of Responsibility in Feeding. In essence, the parent gets to decide what is served, but the child (at all ages) gets to decide how much of it to eat. Excerpt:
The Division of Responsibility for infants:
- The parent is responsible for what.
- The child is responsible for how much (and everything else).
Parents choose breast- or formula-feeding, and help the infant be calm
and organized. Then they feed smoothly, paying attention to
information coming from the baby about timing, tempo, frequency, and
The Division of Responsibility for babies making the transition to
- The parent is still responsible for what, and is becoming responsible for when and where the child is fed.
- The child is still and always responsible for how much and whether to eat the foods offered by the parent.
Based on what the child can do, not on how old s/he is, parents guide
the child’s transition from nipple feeding through semi-solids, then
thick-and-lumpy food, to finger food at family meals.
I think that your method could work somewhat if done carefully. You can have both the dessert and the main course set in front of her, offer alternating spoonfuls, and see how she deals with that. However, if you turn down a demand from her for more spoonfuls of dessert before another spoonful of not-dessert, then you're back to removing control and the risk described above. (And I speak from experience: this is an incredibly difficult process as a parent, largely because it's not how I grew up and I don't want my kid to eat nothing but ice cream.)
If you suspect she's eating less than she really needs at dinner in order to save room for dessert (typically I saw when my kids were older than one, but not much older!), have a few meals without a dessert course. When she's hungry later, offer the leftovers from the entree that she did not finish.
Some additional/related answers that may be of interest: