I agree that the parents should have pulled up their kids over this. That's quite mean behaviour and I would have come down on it like a ton of bricks.

You sound so much like me a year ago. DS was so keen to play with other kids and they all pretty much ignored him and I cried over it too. But with hindsight I can see that he was happy just being in the vicinity of other kids. On rare occasions an older child would tolerate him for a little while and my heart just broke to see how excited he was to get that attention from another kid.

You really don't need to worry just yet. It is normal for 3-4 year olds to not want to play with "babies" and 18 month olds aren't very good at playing with each other anyway, they tend to play alongside each other...it's called parallel play. My DS is 2.5 years and he and all the kids in his mother's group have really only started playing with each other properly in the last 3 or 4 months. There was a kind of transitional phase where they would briefly interact...but now they are all becoming quite chummy...and yes, a bit cliquey too. I notice the difference in how the regulars interact with the ones that are only there occasionally. And my DS is one of the cliquey ones...oh how times have changed.

Having said that, even now DS will get ignored all the time the park etc when he is with kids he doesn't know but although I still hate to see it, I am learning that it doesn't really affect him at all at this stage, especially if mummy is around. Mummy is still Number One lol. (I am a SAHM and DS doesn't go to CC).

ETA: Take the bull by the horns and suggest an outside playdate with someone from your playgroup. Probably most of the other mums are too chicken to suggest it! Even admit that you want your DD to have some more socialising and tell them what you have told us. I bet other mums there have the same worries. I did this with my MG, told them I was worried about DS' socialisation etc and DS now has regular playdates outside MG with some of the kids and it has made the world of difference...they are such good little mates now. Bare faced honesty can achieve wonderful things.