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It's okay to let the baby cry sometimes.
Literal
Sometimes baby [object-を] make-cry-preserve [nominalizer-の] [also-も] good.
Parenting reassurance. 泣かせておく combines the causative 泣かせる ('let/make cry') with the auxiliary ~ておく, which here carries its 'leave something in its current state' sense — 'let them go on crying for a bit.' ~のもいい ('X-ing is also fine/good') is the soft-endorsement pattern, and たまには ('once in a while') tempers the whole assertion. Reads like reassurance to an anxious caregiver: sometimes it's fine to just let the baby cry.