Come on, let's have tea. 
( ( Come on ) ( , ( ( let 's ) ( ( have tea ) . ) ) ) )
(ROOT (S (PP (VBN Come) (PP (IN on))) (, ,) (NP (NNP let) (POS 's)) (VP (VBP have) (NP (NN tea))) (. .)))
Let's have tea.
( ( Let 's ) ( ( have tea ) . ) )
(ROOT (S (NP (NNP Let) (POS 's)) (VP (VBP have) (NP (NN tea))) (. .)))