It's Bed Time Kiddo (Sans x Frisk)
Book One of Undertale Tales
Since everyone from the Underground came to the surface, everything been going well. Sans and Papyrus take cute little Frisk to live with them. But with a jack crazy turn of events, everything they know will change. Lives will be lost, people will change, and will the last words spoken to their loved one be, "It's bed time kiddo"?