Those blanket sleeper suits from target, also come in much much bigger sizes(up to a 6-7 i think it is).
I have one for each of my children except for the eldest who misses out on their sizing, as they are all wriggly kids who kick off blankets, then wake up at night in winter crying they are so cold, then want to climb in my bed and out their frozien feet all over me,, EEEK!!