"We are reading the letter about her grandma and your grandpa."
Translation:Čteme ten dopis o její babičce a tvém dědečkovi.
It sounds a bit weird. Czech national corpus has only 3 cases of "o dědečku" out of 5 billion database entries and all of them are names of some (fairy) tales. "O děděčkovi" has 270 entries.
"Po dědečkovi" has 1820 cases while "po dědečku" has 5. So on one hand it is not impossible, on the other hand it is not normal usage and can be confusing.