what’s the protocol for declining playdates?

Yesterday Kembe came home from school with a really sweet note from the parents of another boy in his class.  I’ll call him Aidan.  The note said that Aidan talks about Kembe all the time, and really wants to have him over for a playdate.  Kembe somehow knew about the subject of the note and is  talking constantly about going to Aidan’s house. My first reaction was to be flattered and excited for Kembe.  He’s doing really well socially and I’m happy that despite his accent and the fact that he looks different from pretty much every kid in his class, he seems to be extremely popular.  (And this isn’t just my observation – the teacher says so as well).  He’s a little charmer and my heart swells that he is well-liked by his peers. But I’ve been putting off calling Aidan’s mom, because I am worried the conversation could be awkward. You see, I don’t know Aidan’s mom.  I’m pretty sure she is a lovely person.  But Mark and I have always said that we wouldn’t let our kids have playdates in the homes of people we don’t know.  I didn’t think we were being that conservative with that decision, but it’s not the first request and it seems like other parents are a bit more comfortable with the idea of dropping their kids off with families that appear to be normal and upstanding.  I don’t want to sound judgmental of people who make different decisions.  I know this is mostly from our own paranoia.  We’re therapists – being paranoid about childhood trauma is an occupational hazard.  I’ve always wanted to keep our circle rather small, in terms of the adults that have access to our kids when we aren’t around.  I would imagine this won’t change much as our kids get older.  The thought of sending my kids to homes where there may be access to filter-free internet?  No thank you. Anyways, I’m really struggling with how to choose my wording with this other mom, because I don’t want to give the impression that I don’t trust her, or that I think poorly of her, etc.  How do you politely say, “it’s not you, it’s me?”  How do I say, “we don’t let our kids play at the homes of people we don’t know well?” without sounding like a jerk?  Because I feel like I’m going to sound like a jerk.  I am perfectly happy meeting them at a park or hosting Aidan at our house, but the invitation was clearly for Kembe to go to theirs, so the subject will likely come up. What do you do?  What are your boundaries in terms of playdates at homes outside your circle of friends?  How do you tell other parents about your own boundaries in a way that doesn’t denote judgment?