which do u think sounds best?

Mason Rory - I'm not a fan of Mason, it's really trendy these days and I have a hard time seeing it on an adult. Rory's really cute, I like it in the middle name slot.

Emmett Harkness - Emmett is a good, strong name that ages really well. Harkness is a little too weird to me, and too close to "darkness."

Percy Ian - Percy is such a stuck-up sounding name in my mind, I can't imagine it on a child. Ian's a solid name choice though, and the combination over all flows well.

Thea-Rose River-Song - I'm not a fan of hyphenated names, and definitely not a double one like this. Thea-Rose could be really pretty, but what about just Thea Rose? 

Artemis Christine - I absolutely love Artemis (and only for a girl, I don't know who told people it was okay to think of it as masculine....). I'm not 100% sold on Christine, though, they seem a little harsh together.

Melanie Elizabeth - Adore both names, and I think together they make a great combo.

Sorry, I'm not really a fan of any of them.  If I had to pick, I would say Mason Rory and Thea-Rose River-Song, but it's only because I love Rory so much and find Thea so refreshing.  Why not something simpler, like Thea Rosalie?  Thea-Rose River-Song is really hippy-dippy (not to mention complicated!  One hyphenated name is enough for a lifetime, not to mention two!).  I really love Rory and Emmett, though--I think Emmett Rory or Rory Emmett would be lovely for a boy.  If Elizabeth Thea was an option, I would love to vote for that, as well.

If I had to choose between these without changing anything, I'd pick Mason Rory and Melanie Elizabeth. If I can change them, here's what I suggest:

Emmett Harley, Emmett Harris, Emmett Harvey (I'm guessing you meant Emmett but typed it incorrectly. Harkness is weird.)

Percy John, Percival Ian (Percy Ian doesn't flow)

Thea Rose, Dorothy Sonja, Thea Solange, Dorothy Rivka (why all the hyphens?)

Artemis Catherine, Artemis Chloe, Artemis Caitlin (I think Artemis is kind of over-the-top, but it's pretty cool and has some cute nicknames. Similar names are Diana, which is the Roman equivalent of Artemis, and Ardis.)

Melanie Crystal, Melanie Sarah (Elizabeth is overdone as a mn)

Mason Rory's very nice!

Percy Ian's cute too :)

I've never heard of the name Emment before and TBH, Harkness made me giggle a little bit (all names ending with -ness do for some reason, I'm not really sure why).

Melanie Elizabeth is my favorite of your girls!

Artemis Christine is very nice but I'm iffy about the name Artemis.

Is Thea-Rose River-Song one name? It sounds like that could be a character from The Host :/ I think it's a little bit too much

boys:

Mason Rory i like this one best

Emment Harkness people will think emment is emmett. which i like emmett. and i dont know what harkness is. sounds like a word.

Percy Ian this is cute.  but i like mason rory sounds more masculine

girls:

Thea-Rose River-Song too over the top.. i love thea rose. as first and middle no -

Artemis Christine  i think artemis sounds masculine even though its the goddess name. and it doesnt really go with such a classic name like christine

Melanie Elizabeth this is cute i like this. so my favs would be mason rory and thea rose. but out of the exact names u listed melanie elizabeth for the girl

I would definitely go with Mason Rory or Melanie Elizabeth.