add children nodes only if node exist
add children nodes only if node exist

luk156/backbone-tree-model,luk156/backbone-tree-model,Contagious06/backbone-tree-model,EnotionZ/backbone-tree-model,kirill-zhirnov/backbone-tree-model,kirill-zhirnov/backbone-tree-model,EnotionZ/backbone-tree-model,Contagious06/backbone-tree-model