As have fun and be happy are pretty much the same idea, how about: "Felix fidelisque esto"? Remembering that Latin grammar, word order etc are not the same as English, that translates word by word as: "Happy faithful and be."
My advice to you - based on past experience of questions involving Latin on AnswerBank - is to check with an 'expert' WHATEVER answer(s) you get here...including mine! For example, if your local secondary school has a Classics Department or even just a solitary Latin teacher, try to get a response from him/her. An alternative is to approach a local Catholic priest.
If someone suggests an online translation site, I'd treat that with even more care than answers here. They are generally much too vague or even ridiculous, unless you are quite knowledgeable about the language in any case.