@Glomoro, again, to me, whose native language has
"T-V distinction" , it's similar to a kid using the "T form" towards an adult. Being
strictly polite, they should using the "V form" in this case (and they usually do that towards, say, teachers or strangers), but it's generally acceptable using the informal "T form" with close family (including granparents) or adult friends, especially if the age difference isn't too big (like, an
oniisan/oneesan, not an
ojisan/obasan). Towards non-family adults it's a kind of sign of trust too, like "I don't see you as a stranger".