I am a professional salary negotiator. I have negotiated hundreds of salaries at hundreds of different companies.
Your goal is to get this new job and also keep your side job. Therefore you should take the course of action most likely to lead to this outcome.
Go through the entire process of negotiating your salary without bringing up your side job. Get them to agree to the salary and other terms that you want. Only after you have reached agreement on everything else (and right before you sign the employment contract), that's when you tell them that after reading the employment contract you are concerned that you might be precluded from this side hobby (don't call it a job) that is entirely unrelated to your work but very important to you. At this point, there is a good chance that they will be so far down the road with you that they will let you keep up your hobby.
Under no circumstances should you mention your hobby until you have a final employment contract in your hands. To do so would be to weaken your leverage unnecessarily. Also do not ask to see their NDA. If they have one, they will give it to you at some point for your signature. That will be another opportunity where you can then bring up your side hobby as if it were something that just occurred to you after you read the NDA.
I have actually negotiated a deal like this before for one of my clients and he followed exactly the steps I just listed above. The employer let him keep the side hobby.