Can I say,

(A) Ali is a ice cream vendor. He is happy of /with his job because it brings happiness to

children. Although he is tired, he enjoys his job very much.

(b) He is happy to see children enjoying eating ice cream. He feels that he brings happiness

to / for them.
Ali is an ice cream vendor. He likes his job because it makes children happy. Although his job is tiring, he enjoys it very much.

He enjoys seeing (the) children eat ice cream. He feels that he makes them happy.
It makes him happy to see (the) children eating/enjoying ice cream. ...

Usually we don't say that someone brings happiness to someone else; we say that someone makes someone else happy.

1) He is happy with his job....

2He brings happines to them...
 CalifJim's reply was promoted to an answer.