I'm definitely showing my age here. My mom used to use white shoe polish on our baby shoes every night, the kind nurses used to use with the sponge on top of the bottle?
What about that? It's made for leather? You could spot test it first.

I'd try the magic eraser. I haven't found anything that my kids can ruin with that around.

Or...now I'm really going crazy. They have stuff at drugstores that takes out hair dye. You could spot test that also. Since you use it on your head it must be safe for leather.