Sorry that you feel upset OP.
I've come to realise, over the years and also through reading here on mn, that friendship is as varied as concept as they come.
Whilst the majority of people, especially those of the same generation, view romantic partnerships and dating in a similar-ish manner, how people view friendships can often differ greatly.
Some people have very long, intense friendships, which they consider akin to family. They seem to be seen positively but can also occasionally be controlling and even abusive in nature. These sort of friendships can appear like a relationship breakup if they fall out, and sometimes people end up not speaking for years if for ever.
Other types of friendships are less intense and leave ample room for different external friendships. You can be part of one group, say school mums, but also be part of a hobby friendship, friends with someone you grew up with etc etc. These groups don't need to fulfil every aspect of friendship. You may prefer to keep some topics/ events for one group, and others for another.
I would suggest trying to view this group in this way. You aren't necessarily going to be a best friend girl group of 4 and do everything together with these women, from discussing worries about your girls to relationship problems, health issues, holidays away. You may do some of these things, but you can have other people in your life with whom you do others.
At the moment maybe they make up almost 100% of your friend time. Often when your children are young this is common, but that will likely change over the coming years.
Nobody enjoys feeling left out, that's for sure. If you had had a fallout or one member in particular doesn't get on with you, then that would be different, but if it's a case of not going to one meet up, then I would forget about it and carry on as usual.
There are multiple reasons as to why a group occasionally cannot accommodate all members, but unless it's done intentionally to hurt someone I wouldn't take it personally.