Be very careful with this. I don't know how but I would suggest getting a test mark on a hidden bit of the sofa and testing - using a cotton bud and see if you can remove this. if you have success with the test one then carefully do the other one. The thing the bear in mind is GENTLE do not use anything harse like scouring powder or brushes as they may remove the ink but leave a ugly scuff mark.
i had the same problem with my car white leather seats in a jaguar completly babbled by a 3 yr old son in biro, i used wd40, it smells a bit but i sprayed that on and it rubbed out
Just spray on some hair spray. It work instantley . i too have a toddler who thinks it funny to write on my cream leather sofa. Believe me it works a treat. :o)