Note sure about your 'rod fixation'. Was this an external splint or an internal rod. I've not heard of an internal rod fixation that was removed after a week. Can you please provide a more clear explanation.

In general I would just get the to do activities with one and both hands that involve opening and closing the whole had. You don't need to make it isolated to the little finger as even in a normal hand this can be difficult. Reassure then this is just take a little time.