You may say, "Anata no otanjoubi wa itsu deshou ka."
Note: please note that when you write it out you are using the hiragana "ha", but when speaking it you say "wa".
あなたの誕生日はいつですか?
anata no tanjobi wa itsu? = when is your birthday
happy birthday
"Your birthday" can be translated as "Anata no [o]tanjoubi."
'(O)tanjoubi.'
Otanjou-bi Omedetou Gozaimasu!
'Otanjoubi WA raishuu desu.'
'Otanjoubi WA itsu desu ka'.
Yes it is.
You may say 'akemashite omedetou.'
Japanese children.
You may say 'ojisan/jichan, otanjoubi omedetou,' written in Japanese: おじさん、お誕生日おめでとう
The Japanese Happy Birthday song is sung the same way it is sung in English but in Japanese's translation. Even in most Japanese schools they also sing it in English version as well.
お誕生日おめでとう O tanjoubiomedetou