What steroid is used for babies?

Depends. Depends upon what for. Treating premature lungs is different than asthma or croup for example. More information is needed.
It depends. Steroids are used to reduce inflammation. In babies with croup, Prednisolone (a liquid form of prednisone) or Dexamethasone are often used to reduce swelling around the vocal cords. In asthma, Prednisolone is often used with a bronchodilator such as albuterol to reduce inflammation in the lungs.