It’s never good to hide that a kid is adopted. It’s one of those things that the kid will eventually discover, and it would be a tragedy if it were never found out. The later a person learns about it, the more painful a realization it will be. That’s the case with many big family secrets. Sometimes it’s age-inappropriate for a kid to learn about the nefarious things their grandpa has done in his past, so their parents keep it from them until they’re in their twenties. That’s all well and good, but kids will notice if they get a bad vibe from Grandpa. It’s better to be honest (in an age-appropriate way) from the time they can understand, rather than letting your kids be confused about how to feel about their grandpa. Your kids will appreciate this transparency much more than being completely shielded from everything that is happening in the family.