Should i be concerned if my 4yrs old baby got her left hand fractured above elbow & plastered. what remedy to avoid infection & odour?

Ensure the cast does: Not get wet, to avoid maceration of the skin. These are usually close injuries (the skin is broken over the fx site), so chances would be very rare. If she starts to scratch due to itching, give her some phenergan, (promethazine) to avoid scratching. Don't allow her to scratch inside the cast with a pen/pencil or something else, as she can then get infected. I use a posterior splint, so arm can be cleaned/dried.