Shaykh ‘Abd al-Rahmaan al-Barraak was asked a similar question and he replied: 

It is not permissible to give them this extra amount, because it is regarded as a bribe on your part to the worker, so that he will give you better service or better food than he gives to others who do not give him this extra amount. The worker does not have the right to single out anyone for better service, rather he should treat all the people equally well. 

But if this extra amount is not like a bribe or favouritism, then there is nothing wrong with it in that case, such as if you intend it as an act of kindness to this poor and needy worker, and you do not intend to visit that restaurant frequently. 

And Allaah knows best.