YOU SAID:
I want to wish them a grand birthday
INTO JAPANESE
彼らに盛大な誕生日を祝いたいです
BACK INTO ENGLISH
I wish them a happy birthday
INTO JAPANESE
彼らのお誕生日おめでとうございます
BACK INTO ENGLISH
happy birthday to them
INTO JAPANESE
彼らにお誕生日おめでとう
BACK INTO ENGLISH
happy birthday to them
That didn't even make that much sense in English.