For the girl: either
Marceline,
Celine or
Marsaili. I think
Marceline would be the best option, with
Celine as her nickname.
For the boy: there aren't that many options.
Chantal doesn't have any masculine forms. If we go with the "song" association, I guess
Carol is an option, but I don't like that as much. If we go with the "stony" meaning, there's
Peter. If we go with a similar sounding name, there's
Chance and
Chandler, and maybe even
Cathal.
I like
Cathal a lot, but I don't think it's very usable where I live, so the best option would be
Peter, which goes okay with
Celine as well (and I don't dislike
Peter at all, so I'm not heartbroken).
Chronic thinker.