You can use '
Ojalá', but it isn't necessary. You may start the sentence with '
Que' and get the same meaning.
I suppose you know that
cumpleaños contains the letter '
ñ'. The word is plural in form, but singular, so the determiner '
sus' is incorrect. If you're writing to a little girl, it would be more appropriate to use the second-person determiner '
tu'. That said, what you really meant to say was '
todos tus deseos de cumpleaños'.
The phrase '
deseos cumpleañeros' could also be used as an alternate to '
deseos de cumpleaños'.
Que todos tus deseos de cumpleaños se hagan realidad. 
Que todos tus deseos de cumpleaños se cumplan.
Que se cumplan todos tus deseos de cumpleaños.
Welcome to the forums, by the way!