Do not use a duvet for the first two years. A duvet is far too hot for a young child. In addition, a duvet can not be properly inserted. Children can also get into the cover because of their trampled up and torment. This is dangerous due to the risk of suffocation.
You can use a blanket in a duvet cover, but the blanket must be the same size as the cover. The duvet cover must be large enough so that you can place it at least 10 cm below the mattress at the bottom and sides. Make sure the cover closes properly. Do not fold a blanket double in a duvet cover.