David finally learned the correct way to say grandpa in Taiwanese this evening. The correct pronunciation is "a-gon", but he has been saying "gon-gon" since he learned the word. Interestingly, both Alyssa and Vivian said "a-don" when they were young.
"Grandma" is a lot easier for them, maybe due to the similarity with "mother" ("a-mom" versus "mom-mom"). All three kids learned how to say this without any problem.