I don't know how much you know about this stuff, but for those who don't know what a "peanut" is, I'm going start by doing a little explaining, is that OK?
Mini Rex is one of the breeds that employ the dwarfing gene to get the small, compact animal described in the breed standard. One copy of the dwarfing gene gets you a rabbit with a shorter body, shorter legs, shorter ears, and a shorter and somewhat rounder head. Without the dwarfing gene (in other words, two copies of the normal growth gene) you get a rabbit with slightly longer ears, legs, back, and face. The MR that has the dwarfing gene generally weighs 1/2 pound to a pound less than the rabbit that doesn't have it. To someone who knows the breed, it's easy to see which ones have it and which don't.
The problem with the dwarfing gene, is that it is what is known as a lethal gene. All showable animals have one copy of the dwarfing gene and one copy of the normal growth gene. Breed two of those together, and you will get some babies that get the dwarfing gene from one parent, and the normal growth gene from the other parent - those are the "true dwarfs." Some will get the normal growth gene from both parents - those are the oversized "false dwarfs". Some bunnies will get the dwarfing gene from both parents, those are called "peanuts." The peanut is roughly 1/3 smaller than the normal sized babies. The head of a peanut has a rather odd appearance - a friend of mine called them "camel babies" because to her, they look a bit like camels. The back end of a peanut looks underdeveloped . The legs aren't twisted, but they are smaller, and peanuts often seem not to be able to control their back legs. The digestive system of a peanut is incomplete, so a peanut cannot survive. They usually die within the first three days.
So, is this a peanut or just a runt? That's a tough call, without seeing it. Two is a very small litter for a Mini Rex. If this is an older doe, she may have issues that would cause her babies to vary in size, completely separate from the dwarfing gene issue. Even people with experience don't usually have good luck at hand feeding (and no matter what you do, you will lose a peanut). If it were mine, I wouldn't make the extra effort, but that's because I've been through this sort of thing so many times. Maybe you'll be lucky, and the little guy will make it.