Automatic display of child value

Hi,

I've created a cascading picklist whereby depending on the parent value, the respective child values are displayed. Now, my query is that how can I display the child value automatically without selecting it manually (In case only 1 child value is there linked to a parent value).

I tried through workflow but then it would be for a specific event only & I want it for new record creation as well as modified records.

Also, please suggest if there is any alternative way to achieve the same.

Thanks,

Sumit