I don’t think there is one absolutely right answer here. It depends on a variety of factors, so I think it’s best if we discuss things in the context of what you don’t want to happen.
For example, you don’t want your kids to learn about the divorce from a source that is not you. You don’t want to lie for long periods of time about what is happening between you and your spouse. But you also don’t want to tell your kids before you tell your spouse since that will put them in the position of having to keep that secret.

Therefore, the best time is somewhere between when you tell your spouse, and well, your kids start noticing that something is wrong. That time window may take weeks, days, or hours, depending on your specific situation.
Speaking of timing, it’s also a good idea to sit down with a custody lawyer before breaking the news to your kids. They will have questions about what happens next, and talking to an attorney will give you concrete answers to those questions.