"A smiling face"

Smiling is present continuous tense; it means a face that is smiling now. So #2 is correct.

Thank you so much and then which one is correct?

1) I said hello to them with a smiling face.

2) I said hello to them with a smiley face.

3) I am going to help them with a smiling face.

4) I am going to help them with a smiley face.

Smiley face (sometimes spelt smily face) nowadays refers to the emoticons you used at the end of your post. So if you don't mean the emoticon, you should always use smiling face. So sentences 2 and 4 are wrong.
Sentence 3 isn't really right either. HELP THEM WITH <noun> implies that the noun is the thing they are doing, that you're going to help them with. For exampl

Thank you, KrisBlueNZ so much for the clear and great answer and I was wondering if in "I am going with a smiling face to help you.", if we analyze the phrase, can we do it like "with a face that is going to be smiling" because of the tense agreement? And in "I said hello to them with a smiling face", it is "with a face that was

I would say that "I am going with a smiling face to help you" means that my face will be smiling.
"I said hello to them with a smiling face" means that my face was smiling when I said hello to them.
That's as clear as I can be.