If you're using .minerva, there's an object called "children" containing an array with -I guess-, all the children you had currently in the nursery. After checking with one of my save, I think RaceType = 23 is the tentacle beast. Logically deleting the whole array entry should delete it from your save. You might also need to rename the other entries to fill up the gap left in the array. Now, it's only speculations from my part and even if I'm right about what to modify, it might fuck up some things. So be safe, do back up before trying anything.