implement method to get all child nodes under a given node
implement method to get all child nodes under a given node

CompEvol/beast2,Anaphory/beast2,Anaphory/beast2,tgvaughan/beast2,tgvaughan/beast2,CompEvol/beast2,CompEvol/beast2,Anaphory/beast2,Anaphory/beast2,tgvaughan/beast2,CompEvol/beast2,tgvaughan/beast2