Your 1 month old duckling and 1 week old should not be together all day unless they are separated and where they can see each other the Size difference alone can cause injury and pain as you’ve already found out. still in a brooder separate them so they can see each other give your small one a couple weeks to get strong enough to handle the1 month old better. Once the duckling is over its injury try placing them together a few times a day with you right there to separate if needed. Hopefully rest and good feed and some poultry vitamins will get your little one back to normal.