Children must  do their own work. 
( Children ( ( must ( do ( their ( own work ) ) ) ) . ) )
(ROOT (S (NP (NNP Children)) (VP (MD must) (VP (VB do) (NP (PRP$ their) (JJ own) (NN work)))) (. .)))
Children are responsible for completing their own work.
( Children ( ( are ( responsible ( for ( completing ( their ( own work ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Children)) (VP (VBP are) (ADJP (JJ responsible) (PP (IN for) (S (VP (VBG completing) (NP (PRP$ their) (JJ own) (NN work))))))) (. .)))