It is not the first time that Mary has met the Queen. I believe they did meet in private for The Prince of Wales's 60th birthday dinner/concert hosted by the Queen in 2008.
The Cambridges have only met them for once on public engagement plus it was their first time being at Ascot, so they were arranged to sit with their friends whom they can closely refer to rather than those they do not know very well.
I don't know about the Cambridges, but William would have met them at the same 60th birthday (I assume he attended) and I have seen a photo of a young William with CP Frederik and then CP Willem Alexander at some event in London. Not sure it was London, but anyway. I think they wore a tux.