That's a great photo, we've never seen the Queen of Hearts there either, again, just Mad Hatter and Alice. The time the girls were asked by Alice to play with them it was just the two of them. We've seen it happen a few other times as well (we've been quite a bit.....lol) and never seen Peter or the Queen. That obviously doesn't mean it's like that every day, but that's been our experience.
I just don't want anyone to assume that Peter will be there and be disappointed when he's not.
I've only seen this happen when the weather was good, as far I can remember I've never seen them playing this when the weather is bad. IE, raining.....if it's cold sure, but not rainy.