Elven genetics

[Legolas]

Are the children of a union of an elf and a man mortal or immortal? I think it says somewhere that Aragorn and Arwen's kid was mortal.

Seems you missed the conversation altogther - Arwen was not an elf. She was half-elven - and was granted a special choice by Eru as to which race she was to share her doom with. This choice was given to seven people only - Earendil/Elwing's family, only because of Earendil's great deeds. Earendil and Elwing, their sons Elros and Elrond, and Elrond's children Elladan, Elrohir, and Arwen. They were *not* elves - they were half-elven.

This is an often discussed/confused concept, which is why this site has provided a full explanation here .

All other half-elven beings (of which we know only two other instances) are mortal by the fact that they have some mortal blood (Dior and his sons, and the line of the Princes of Dol Amroth). These are the only other occurrences.