Grandpa and Grandpa

No, my family has not gone "alternative lifestyle".   

My dear little Hayden has an issue.  We have been trying to get him sorted out.

Whenever Hayden wants to go to my parent's home, he asks to go to "Grandpa and Grandpa's house".  Today while we were going to pick some oranges, this is the following conversation.

Hayden:  Are we going to Grandpa and Grandpa's?
Me:  No, we are going to GrandMa and Grandpa's.  Hayden, say GrandMa.
Hayden:  GrandMa
M:  Now say GrandMa and Grandpa's
H: Grandpa and Grandpa.
M: No, GrandMA and Grandpa.
H:  Grandpa and Grandpa.
M: Say, 'We are going to GrandMa's house.'
H: We are going to GrandMa's house.
M: Now, 'We are going to Grandpa's house.'
H: We are going to Grandpa's house.
M: Say, 'We are going to GrandMA and Grandpa's house.'
H:  okay, we are going to Grandpa and Grandpa's house.
M: No... Say it again.  Where are we going?
H:  I DON'T KNOW! 

What do you say to the poor kid?  All we could do was laugh.  Poor kid... so confused