Sorry to be blunt, but if I were your sister, I wouldn’t have a b-day celebration on Good Friday, even a child’s b-day. I would move it to Saturday and explain to my child why. This would be good witness to my child about what’s more important.
If I were you, I wouldn’t go to the dinner. Good Friday is not just about how much we are allowed to eat; it’s about the whole approach toward the day - the day our Lord died. I know some Catholics go out to eat on Good Friday like it was just another day of the year, and order fish, but I personally believe this misses the whole point.
To answer your question though, no, I wouldn’t eat dessert should I somehow end up at the dinner in the first place.
