Well, like your son, we got milk flick's on our leather sofas too... dried milk flick spots.
I just used a diaper wipe, and rubbed rubbed rubbed, and it did wipe off.
And then I used Lemon Pledge... which I figured is 'moisturizing'. (ha, ha... yah I know not the 'standard' way of cleaning leather. But it did not harm it. I just did it on the fly as I was cleaning the living room and was multi-tasking! LOL).
I don't know what type of leather 'texture' your car seats are... but our leather sofas are 'glossy' finished. Not, raw leather or suede.
Or, you can use 'saddle soap.' Its found anywhere where you would find shoe polish/cleaning supplies. Which come to think of it.... try any type of "shoe" product... since shoes are leather too. I've done that to our sofas too... it's fine.
Don't use any harsh chemicals on leather... then those products might stain it too.
Good luck,
Susan