When trapping kittens, make sure you are using an appropriately sized trap, like a Tru-Catch 24 or Tomahawk 104 trap, or any trap made specifically for kittens. Larger traps, like those used for raccoons or tomcats, are too powerful for kittens and those traps can put them at risk, and sometimes the kitten isn’t heavy enough to trip the plate.
Alley Cat allies suggest that you prop open the trap door with a water bottle or other similarly sized object (like a stick) attached to a string, so you can spring the trap manually when all kittens are safely clear of the door. Once the kitten is fully inside the trap and clear of the door, pull the string hard and fast to remove the water bottle.
One Per Kitten
Make sure to set out at least one trap per kitten, to discourage kittens from following each other into the same trap. (They may still do this, but springing the trap manually will make sure no one gets caught in the trap door) If you do catch two kittens in one trap, either use an isolator to transfer one into another trap, or bring an extra trap to the clinic and the clinic will separate the cats after surgery.
Trap and Neuter
As cat experts, Alley Cat Allies understand your reservations about interfering with nursing mothers and their kittens, but the best thing you can do for the family in every situation is to trap and neuter them as soon as it is safe to do so. Where you place the kittens after trapping—either in adoptive homes or back with their colony—depends on many things, like your own time and resources. You can read more about that here.