What can be done for a bicuspid tooth in 9yr old?

Depends on : The problem. If just decay then a filling should do. If deep decay then perhaps removal of some of the decay with a sedative dressing being placed to allow the root to finish formation and in 6 months a final filling with removal of the remaining decay, sometimes a root canal with apexification so the root finishes forming. As always brushing is paramount.

Bicuspid: I am not quite sure what the question is? The bicuspids or premolars are permanent teeth and usually come in between ages 10-12 years old.