Who's liable for repair costs? Tenant's child accidentally leaving bath tap on...

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Our tenant's child accidentally left the bath tap on, which overflowed causing water damage to the ceiling below (buldging plaster and staining); and water damage to the carpet in the room below. The tenant is insistent that they are not liable and since the ceiling and carpets are not 'contents', that the landlord is liable to fix under landlords 'buildings' insurance. Who's liable for the repair costs? What contribution is legally and/or reasonably expected that the tenant will pay? Thoughts?