I'm sure the site is legitimate, but it's accurate to observe that this article in particular is based on an online poll with self-selecting participants. It's just not a great way of doing a poll. That doesn't mean that broad observations can't be made from it, e.g. that some physicians are vaccine-hesitant with regard to this vaccine and 5-11 year olds, but the methodology is completely inadequate to say what proportion that actually is with any confidence at all.
I would regard that kind of poll as inadequate for this purpose, and consider it unprofessional to use it as such.
And it's also accurate to observe that anyone can register for free on medscape, it's not hard to access at all.