We had a six children (including ours) in our house over the weekend and one of them found our return address mailing stamp (we use this one, which has served us well for several years so far). The kid did what kids do, our wall got redecorated, and I got the opportunity to do an experiment.
I found a wide range of advice online and decided to go with the recommendation here to (1) apply white toothpaste, (2) let it sit for 10 minutes, and (3) wipe it away with a wet white washcloth, repeating if necessary.
I was quite surprised to see how well this worked. I did not need to repeat the process; the ink was completely removed after just the first round of steps.
The toothpaste fully absorbed the ink, so wiping the toothpaste took the ink with it. As far as I could tell, the toothpaste wasn’t serving as an abrasive (which is how dry toothpaste is sometimes used in car scratch repair).
The toothpaste I used was Colgate Total Advanced Whitening, which was just what we happened to have around.