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Locking a young child in a nursery is a cruel act.
Literal
Young-child [object-を] children's-room [in-に] shut-in-[maintain-ておく] [nominalizer-の] [topic-は] cruel [attributive-な] act is-[である].
~ておく ('do X in advance / leave X in a state') is the productive auxiliary for maintained states. 閉じ込めておく = 'keep [someone] shut in / leave [them] locked up.' 残酷 ('cruel') is a na-adjective. である is the formal copula. Ethical/legal register.