For some reason, mass manufacturers cannot make a mitten for little kids (or big ones, sometimes) with a long enough cuff. Maybe if you don’t move, the cuff is long enough to keep you warm. But if you move (and who doesn’t?), there is always that little strip of wrist that gets exposed to the cold — the jacket sleeves creep up and mitten cuffs are too short to cover that little area of skin. A wrist warmer/gaiter does the trick. My oldest confirms this when she works the drive-up at the grocery store. Since I used dark yarn, I put my big hand (compared to a three year old) in the cuff so you could see it better.
The neck gaiter is for another little boy. I put a button on it to keep the ends of this short scarf together. The parent can put the button through the scarf wherever there is a hole — which is all along the garment. So the garment can adapt as the child (or parent) needs. I myself struggle with cold air down the neck, to varying degrees, with jackets and coats.