Maybe it is standard English, but "I've seen them worse." does not sound like it to me. There is something peculiar about it but I cannot seem to get a fix on why or how it is different.
Does it have to do with them, worse or maybe both?
Minnie May has croup all right; she's pretty bad, but I've seen them worse.
Source: Anne of the Green Gables: Chapter XVIII -- Anne to the Rescue