He blinks a little at 'kid,' before giggling. Well, yes, he sure does look young, but... he likely is several thousand years older than Joker!

"Oh, I would make a terrible hostage. Terribly annoying when I wish to be, you see."

Which leaves only the other option, though admittedly Hythlodaeus doesn't know what kind of 'job' he's supposed to be working here; but Joker's casual talk of hostages doesn't have him hopeful that this is a very virtuous job. What kind of person has he stumbled across this time?