That's it in a nutshell really.
Grim playdate today with alpha-mummy of DS1's alpha-friend plus alpha daughter.
Alpha daughter is lovely and very kind to DS2 (nearly 6 but raw, very raw, social skills).
But against all my planning it ended up being a meet in the park. Cue usual struggle between the two usual extremes - either poor DS1 trying to make others play with DS2 or me having to play with DS2 while other mothers chatted.
I ended up having to leave with DS2 with alpha mummy left looking after DS1 for the umpteenth time.
sorry to moan, I know I am bloody lucky that the older child has the skills to shield/help the younger child and is willing to do so. But I'd resolved never to stray into alpha-crowd playdate land again without bringing a playmate for DS2 (he has a couple) and felt tricked!