In reply to BazVee:
Adult bag tied off to kill any dead space longer than the child (be careful that the bag can't suffocate the kid)
You can also get baby sleeping bags for tinies, even very good down ones if you want to spend that much.
Otherwise I'd insulate well from the floor with an adult bag folded up on top of a mat, then use the child's normal blankets etc (for reassurance) with an extra layer or 2.