And yep.. Most of my kids sheets don't reach across them to be tucked in, found the same with the cot. Never understood it.
My son had problems staying under his blanket and would wake up frozen in the morning. He has a king single bed, but we've put a queen doona on there and tucked it right under the sides of the bed. Perhaps the next size up sheets would work better too. Shame they come in sets of fitted and flt though because the fitted would go to waste if you got a set.