from collections import deque
from operator import itemgetter
 
def convert_tree(lst):
    tree = {}
    queue = deque([(lst, tree)])
    while queue:
        entries, branch = queue.popleft()
        for key, children in map(itemgetter('key', 'children'), entries):
            if children:
                queue.append((children, branch.setdefault(key, {})))
            else:
                branch[key] = ''
    return tree
 
lst = [{'key': 'a', 'children': [{'key': 'a1', 'children': [{'key': 'a11', 'children': []}]}]},
       {'key': 'b', 'children': [{'key': 'b1', 'children': [{'key': 'b11', 'children': []}]}]}, ]
print(convert_tree(lst))

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