At the stage I'm at right now (with a preschooler and a baby), my kids' needs are so intense and parenting things pop up so frequently that I don't think I could see a therapist who didn't have children. There are things that people without children think they know about how to raise children that can be completely at odds with the actual lived experience. If I'm likely to get into tricky/complicated topics, I need to know that they know firsthand what they're talking about. Some of the worst things that happened to me happened when I was quite young and there are unpleasant reminders that pop up indirectly because of my kids' developmental stages, so the parenting stuff and the trauma stuff can get blurry for me. That's a big part of why I need a therapist who has kids at this stage in my life.

That being said, I've had two therapists who I have been close to and who helped me make big positive changes in my life (as sort of mother figures). One has kids and the other did not, and they were both excellent in their roles. So I don't think somebody needs to be a parent in order to help with attachment issues. Many things that therapists are trained to do are also the things that good parents do for their children, and the therapeutic relationship has been really helpful for me in working with attachment issues.

So the bottom line for me would be how much I think practical everyday parenting stuff might come up vs. how much of my own attachment needs might be called forth for examination with this person (and do I feel safe with them). Like Scarlet said, it really comes down to personal preference.