OF COURSE you feel 10 years old. These hurts hurt. The bloody do. They hurt when we are 10 and they still hurt when we're 20, 30, 40.
to you.
I can't tell you why they did it, but hope this is helpful:
Try not to take it to heart (easier said than done, I know).
You can not rush a friendship which grows out of a casual acquaintance. You just can't. It takes it's time. Be patient, and looking as though you are bothered or upset won't help.
Perhaps these people also know each other differently to the way you know them (i.e. maybe their DC have a shared hobby so they see each other extra to just the ways you see them), so they have connections you don't share (yet).
It really could just be a coincidence.
It's almost certainly unthinking, and not mean (it could turn out to be calculated and nasty, but you've got no evidence of that at this point, so don't treat it as deliberate).
Say things like "oooo, that sounds fun, we'd have loved to have done that" but without putting any pressure on.
Continue with the invitations you've issued - that's a good thing, and the more you do it, the more integrated you will be and the more likely they are to invite you next time.
DON'T SHOW THEM YOU ARE HURT!!!!!
Try and remind yourself of all you have OUTSIDE this group - other friends, family, relationships.
Sometimes friendships we want to happen grow and blossom, sometimes they don't. Try and move on if you can if it doesn't, and try and accept this for what it is.
Would it help for you to casually drop into the conversation some fun/sparkly things YOU'VE done? That they might say "ooo, we'd love THAT" to?
Here's a couple of stories:
For the past year and a half, I've been doing regular exercise classes at my local gym. I've got to know lots of people, in particular a regular group of 3 others. I'm now definitely "part" of their group. We chat. We've been out a couple of times, we get equipment out for each other, we hang around together. The three of them have each other's mobile numbers and call/text to say they are or aren't coming to a particular class. None of them has mine, and it's never been suggested we swap numbers, even though they miss me if I'm not there. I've said NOTHING. And won't. I'm biding my time. ONE DAY it will have been of benefit to one of them to have had my number, and at that point, we'll prob swap numbers. Until then, I'm not going to say a word. And I'm not going to be upset about it (they've known each other for over 5 years, and me for less than 2. It's not personal and they're not mean - I know they're all fond of me, and enjoy having me as part of "their" circle).
When DD was in reception, a new girl came to the school. DD LURVED her. She wanted to be her bff. Unfortunately, the girl had a bff already - the younger sister of her/the other gir's older sisters AND the mum's were friends too. You could not have got a more tightly knit group. To be clear, that's 3 friendships: The mums. The two older sisters and the two younger ones, and my DD wanted one of those younger ones to be her special friend.
I fucking stalked the mums on the playground. I always stood near them. I ear-wigged and joined in on their conversations. I was HELPFUL. I was perky. I laughed at their jokes. I said "oh, me too" a lot. I was always careful to invite BOTH the youngest ones round.
DD is almost 14 now. The one she stalked IS her special friend, but the other friend is part of the same (very close) group of 5, and their friendships are brilliant, and I'm (proper) friends with both the mums too. It IS possible to break into a clique.
I've outed myself enough already, and this is VERY long but my last story is I started a new job a couple of years ago (almost) and a VERY established member of staff was tasked to work with me on my very first project. She HATED the project (which was something I'd brought to the post from previous work) and HATED me, and spread the word. And not quite everyone, but LOTS of people listened to her and believed her. People I'd never met were rude, obstructive, unhelpful, nasty, etc. I was the person it's acceptable to hate. Things were made very, very difficult for me. (luckily, I just decided not to care and just got on with the job and reminded myself that work is work and friends are friends, and it didn't matter. But it hurt a lot that people WHO DIDN'T KNOW ME thought it was ok to hate/be rude/be nasty to me and not help when I needed it. I thought it was one thing if people knew me and didn't like me, but it was hard being judged by people who DIDN'T know me. Maybe in some ways it was easier, because I KNEW they didn't know me - some of the people who were mean had never spoken to me). Then, after a year being blanked, and new member of staff joined the team. And she (independently) came along to some of my exercise classes (see above!) and we bonded. Properly bonded, and very quickly made a proper friendship. She's very lovely, and people at work like her a lot. And she (not knowing the history, which I certainly hadn't mentioned) talked about me quite a lot at work, and about what fun I am and how much she likes me. There's also been a change in the management structure, and in several key areas, I have the most knowledge about stuff, so they now rely on me for several things.
OMG how things have changed... I'm now "courted" by almost everyone. People fall over themselves to help me. I am invited out on socials (and on small socials, that only a few people get invited to). My mum keeps saying "you SEE!!!!" very jubilantly whenever I tell her the latest. People creep to me! People tell each other about how they're close to me.
I'm just 
about the woman who spun against me in the first place, who I'm nice-but-distant to, and who keeps telling anyone who'll listen how much she likes me now!
The lesson is - things can change.
BTW, if this post makes me sound super-popular, super-sorted, I'm definitely not. I've got less friends than most people. Like, less than 20 on FB. I'm not a hugely popular woman. (except in this workplace, apparently!) (I keep thinking, once they get to know me, they'll discover bitch-woman was right all along, but I'm enjoying their not knowing. The change is nice!)
Good luck, OP!