Is not wrong to let them know you doing freelance job. You can elaborate more why you like to do freelance job as additional with current job you had. You can show to interviewer between patience and hobby as most of time freelance is part of our hobby too. Hope this workable to you. :)

But like for me, I don't dare to tell the company. Because not all company ok that their employee work freelance or part time right?

Hello, there is no problem telling the interviewer if you have part time job as long as both of the job you can bring a good result, so it shows that you're a good multiskill candidates!