My experience with therapists is that if they think your kids are in danger, they have to call CPS, but that they aren't going to think that kids are in danger from a parent who reports being tempted to slap or spank. I think most therapists would be really unlikely to call CPS even if you came in and admitted that you lost control and slapped a child and you need help to make sure it doesn't happen again.
Anger management is a very common issue for parents. Children don't stop being needy just because you're at your limit, and they don't stop pushing the boundaries just because you can't take any more. So therapists see a lot of parents who want help processing the emotions that their kids bring up (lots of us have baggage from our own childhoods that we only see when our kids remind us of it), and putting together strategies for remaining calm and reasonable.