Honestly, I read that more as a cartoon moment than a "try-this-at-home"
...but, for curiosity's sake, I expect it would be like making pretty fruit-filled ice cubes for punch: Freeze the Mentos first on their own (like spread them on a cookie sheet or something) then fill your ice cube trays and keep a close eye on them, adding the already frozen candy when the ice was almost solid.
(Mind you, I still don't think the Mentos would be totally hidden. - Like I said, I wasn't taking it as a serious plan.)