Often when I go out for a meal with my mother and grandmother, we often go to restaurant with large side dishes and also large main courses, as a result I am usually full and don't want a dessert. Owing to the nature of the restaurants (Pizza Hut, for those of you who know it), we typically order a dessert after having the main meal.
Usually after we finish the main meal my mother/grandmother offers me a dessert of my choice, and I usually say no, however they keep saying "oh go on", and eventually I say yes, usually because I realize that if they eat without me, that may make them feel bad, so I usually agree to get them to order me a dessert, and then don't eat it.
While answer specific to the people I'm talking about (parents/family) a general answer for non-family would be far more appreciated (hence why I'm not using the family tag, as that's not the category of answer I'm looking for). So, how can I politely say "no" to being offered a dessert, but also preferably in a way that they won't then feel awkward eating without me.