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 must never do their own work, it is up to their parents to do it for them.
( ( Children ( ( must never ) ( do ( their ( own work ) ) ) ) ) ( , ( it ( ( is ( up ( to ( their ( parents ( to ( ( do it ) ( for them ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (S (NP (NNP Children)) (VP (MD must) (ADVP (RB never)) (VP (VB do) (NP (PRP$ their) (JJ own) (NN work))))) (, ,) (NP (PRP it)) (VP (VBZ is) (ADVP (RB up) (PP (TO to) (NP (PRP$ their) (NNS parents) (S (VP (TO to) (VP (VB do) (NP (PRP it)) (PP (IN for) (NP (PRP them)))))))))) (. .)))