Since you say you need only an immediate remedy, I hope this means that you intend to take the baby to a doctor for a check up in the near future. Most contusions are minor and do not really require treatment either immediately or later, but some can be quite serious, even causing blood supply to be obstructed and leading to a coma or death. This of course depends on the extent and severity of bruising and on the location - contusions on the head or near vital organs can have serious complications. This is not to scare you, but to remind you of the possible seriousness of the situation, especially since the patient is an infant.
For a minor bruise, the normal approach is to rest, apply ice and compression, and elevate the affected part of the body. This method of treatment is known as RICE - rest, ice, compression, and elevation. However, for a baby, not all the parts of this treatment method are possible or advisable. In fact, if your child is particularly young, you will not be able to do any of this - it is difficult to prevent an infant from using a limb or to elevate the limb; ice is not advisable, and bandage for compression will be difficult to maintain as the infant will be uncomfortable with it. For a severe contusion, there is no immediate remedy except for rushing to the doctor.
answered by G R