My own view is that I would go to a professional for advice on specific expertise, and they may have developed that expertise in the context of having their own children or from years of working with children (e.g. teacher, midwife, nurse or doctor, child psychologist). Either way, they are usually experienced enough to give good advice (or at least the bad advice is not limited to non-parents). The problem often comes when you get an inexperienced trainee, say, who doesn't have children. I had a trainee midwife like that and she was just awful, she simply had no idea what it was like to give birth (so kept saying completely daft things) and hadn't enough experience of watching others to realise how daft she sounded. With a few more years of experience, she may have been great. I was thinking of that lovely, slightly old-fashioned midwife on One Born Every Minute who didn't have children but had made midwifery her vocation, she was just so reassuring.
However, there's a big difference in getting advice from a teacher on behavioural issues, say, and those times you want a sympathetic ear or someone to empathise, in those situations, someone who has been through a similar experience is great as you are starting from the same starting point.
So, to me, it's about how much experience the person has (regardlesss of having given birth or not), what type of person they are anyway, and what you want from the situation (being told your child is hyperactive when they are just a normal two year old running around on the beach, for example, is just what you don't need from a childless relative, however you may be delighted to get a formal diagnosis of hyperactivity if you needed one from a consultant who didn't have children themselve but a wealth of experience).