DD (10), goes to our local primary school. She has SN which include violent outbursts and antisocial behaviour. We have met with the school's SENCO loads obviously. She (the SENCO), is lovely but as she often ends up being the only person qualified to deal with DD (statement issue), she has seen DD at her absolute worst over a period of a few years. She has also heard us talk often about how hard she is to manage at home etc.
DS (6), goes to a different primary school which is also nearby. He is a friendly and sociable little thing and is asked on playdates - we usually explain that it's hard to reciprocate and then once every couple of months invite 3 or 4 of his friends out to the park, cinema etc after school while DH is with DD. We tend not to explain exactly why we don't do playdates at home unless specifically asked. We have made up stuff in the past 
Lately DS has been coming home speaking about a particular classmate, who is collected by a CM. I approached the CM and passed on an invitation to a playdate and the next day the CM said that was fine. I asked if I should take her contact details or the parents' and yesterday she gave me the mum's phone number as we'd be having the DC past the time they stayed with the CM.
It turns out that the DC's mum is DD's SENCO. Today she texted to say her DC couldn't make the playdate after all.
AIBU to think that it might be to do with not wanting her DC around DD? What should I do? I did tell the CM it would be a park playdate as the weather's so good but don't know what got back to the mum.