Years ago, my son was autism was about 7, about to be 8 and we decided we were going to get 'him' a puppy. (Us, really, of course, but we were aware of how much dogs help some kids with autism relate to the world and also our younger son was phobic about dogs so we wanted him to see dogs weren't really scary).
We never ever discussed it when the kids were even in the house. (Husband had a job that started at 4 AM and so was back in the house by mid day when the kids were at school).
We saw a chance to get a pup but again, decided not to mention it, or even hint about it, at all in case it fell through and we ended up being disappointed. Son had something else lined up for his birthday and no idea we were even thinking of a dog (especially as he knew his brother was scared of them).
We were going to see this litter of pups but in the days before the interwebz, so hadn't seen a picture and didn't even know the pups' colours. We just wanted one of this breed.
That same week, there was a Parents' Evening and his teacher said "Son 2 has been saying the weirdest thing... We all got in the story corner, and started talking about (something I forget) and he started telling everyone he is about to get a black and white puppy. Is it true? He's so excited!"
Everything went well and the pup was actually 8 weeks old on son's birthday so dad 'popped out' for a bit and came back with our lovely black and white pup. The whole litter were black and white.
I know, beyond a shadow of a doubt, we had never even talked about it when any of the kids were home. He had no way of knowing. We hadn't even casually mentioned getting a dog. It would have been utterly out of the blue, to the kids. Especially as son 3, then in nursery, would scream "I'm scared of the doggy!" and cry when he passed one in the street.
Oh and it cured Son 3 of dog phobia as well! Two years later, he got his own dog as well.