gistfile1.txt

var tree = Immutable.fromJS(
  {id: 0, children: [{id: 1, children: [{id: 2}, {id: 3}]}, {id: 4, children: [{id: 5}, {id: 6}]}]}
);

function reduceFn(list, item) {
  var children = item.get('children');
  list = children ? children.reduce(reduceFn, list) : list;
  list = list.push(Immutable.Map({id: item.get('id')}));
  return list;
}

var ids = Immutable.List.of(tree).reduce(reduceFn, Immutable.List());

console.log(ids.toString());