While I can't answer your question about client confidentiality with accountants, I would suggest you review any policies your company has about moonlighting, and take appropriate action based on that.
For example, many company are okay with employees doing other ventures outside work, assuming: 1) it doesn't conflict with the company's business, 2) it doesn't interfere with your work, and 3) you don't use company resources. If that's the case with your company, why not just disclose that you're doing your side project?
Of course, if the company doesn't allow moonlighting, then I guess it's up to you if you want to take the risk with the accountant. Perhaps you could just ask him/her if they'd keep that info confidential?