I've never had slings refuse pre killed. Some juveniles may not take dead prey, others will. Just leave the dead feeder at the mouth of the burrow overnight and if it's gone by morning you know it ate, if not take it out.
I leave prekilled prey items in at the entrance of my OBT's burrow, and it usually drags it in over night. You should try it out and if it doesnt work then you've really only lost 1 prey item so its not a big deal.
My slings eat pre killed no problem. I try moving it around with the feeding tongs to see if they will pounce it or not but not yet.
They do however push the carcasses out of their spaces they are really good at cleaning lol.