So sorry you lost your chicksWeasels and rats can squeeze through impossibly small openings and do the kind of destruction you've been seeing.
Besides setting traps, I would spend a few hours locating all vulnerable spots where openings and cracks two inches or larger exist and blocking them by screwing sheet metal over them. Sheet metal in rolls that can be cut to size with sheet metal scissors are available at any hardware store.
I like bucket roller traps for rats. Easy to set up, safe around chickens, and rodents are easily disposed of by simply dumping them out of the water in the bucket. Other predators aren't as easily trapped, and predator proofing your run and coop is the best way to deal with them.
