I don't think Node can contain a std::vector<Node>, since STL containers cannot have incomplete value types. But maybe I'm misunderstanding you?

I think absl::flat_hash_map<T, std::unique_ptr<Node>> would be worth considering in this application. Keys and values are stored inline, so the memory and creation costs should be comparable to std::vector - https://abseil.io/docs/cpp/guides/container#abslflat_hash_ma...