You'll want to test this is a discreet place first (because it may alter the colour)... but aerosol hairspray, aerosol furniture polish and butane lighter refill will all work (it's the butane, not the hairspray or the furniture polish that works)... that's a solvent for most inks. The problem is that it's a solvent, and may damage the leather.
A friend's son did this with a Sharpie marker on their leather couch. She got to confront her hypocritial response in a very uncomfortable way, being a person who is very much 'above all that' when it comes to thinking material things are important.
Another friend's son, when he was in kindergarten, wrote his name with a rock on the side of their brand new car. His name is Ed or Rob or something, and his parents were horrified... but not quite so horrified as they were when his little sister decided to do the same thing to 'her' side of the car -- her name is Alexandria. Nice paint job... ha ha ha! They didn't bother getting it repainted, expecting to own it for a long time, by which time they're hoping it will be scratched or dented or wrecked by some bad driver, so it won't affect the value when they sell it.
Of course, little kids are so helpful - they give us all kinds of physical instructions like 'don't leave pens where I can reach them when I'm in the car...' <grin>