Isopropyl alcohol (rubbing alcohol) will take the ink out. We had a similar issue when our 2 year old took a red ball point pen to our light gray leather seats earlier this year. It took several applications and lots of elbow grease, but it worked.
I'd check in the owner's manual to see what it says about caring for your leather seats. If not, I'd call the car manufacturer and ask. If it's under warranty, and something happens, what you use may render your warranty negligible.
Usually, we take a baby wipe to the seats when we see foot marks, etc on them, and it does the trick. I've never had to use anything else.