Cat is hissing at small kids?

He doesnt do it to anyone else, older kids 13+ or so he doesnt but small kids he seems to hiss and get scared. I do not know why, I do not have any small kids in my house living here but people bring theres over occasionally. Usually he just goes and hides if they come to close so not a big issue but i am just curious is there any method to get him to stop hissing at them?