Christian Tiago and
Matias Alejandro are great. I think I'd go with
Christian as that's the way people are naturally going to spell it.
Tiago seems like a good idea for you.
Matias is really nice, although I prefer
Matthias. Obviously that spelling isn't an option though, which is cool. I think it fits in well with the other kid's names and I think you will grow to love it, even if you're not 100% sold on it yet. There really isn't anything terrible about it at all imo.