Concatenating list and tuple (or tuple and list) you can write as:

grandchildren = [*node.children[0].children, *node.children[1].children]

The only list methods not available for tuples are mutating methods (using them with a shared empty list would be a bug), and copy() (a[:] and list(a) work with both lists and tuples).

>>> sorted(set(dir(list)) - set(dir(tuple)))
['__delitem__', '__iadd__', '__imul__', '__reversed__', '__setitem__', 'append', 'clear', 'copy', 'extend', 'insert', 'pop', 'remove', 'reverse', 'sort']