Thanks, glad you like it!
It's trait-based and partially relationship based: you're looking for an alien who's the type of person to willingly teach a small child how to erase memories (maybe evil, maybe irresponsible, maybe understanding of the desire for more knowledge than might be considered appropriate, etc).
If you have my premades there is at least one who'll do it, otherwise you'll need to stumble on someone by chance. If the sim doesn't like you they won't help, though.