I've heard this before, so I request all my family member (including my domestic helper) to ask my daughter as "Kei Kei" or "Hoi Kei".
Last Sat my domestic helper asked Kei Kei to change diaper, she shouted out "Hoi Kei, change diaper, Hoi....." then my daughter reached and answered "Kei....."
I felt very surprised because even till now she never ask us as papa or mama, and she still present or express herself in baby language.....