A lot of people go to the group with someone they know, or lots of people they know. They don't necessarily meet there. It's rude to call people "a clique" when they're just a group of friends talking. If you saw three women on a bus having a chat about work you wouldn't deem them "cliquey bitches who won't talk to me."
That said, I don't really think they're all that great for adults looking for someone to talk to. Of the few who do go alone most will keep to themselves. At my baby groups the solo mums tended to stick to their phones or a magazine. They wanted some peace and quiet, not a chat.
Or, they may indeed have seen each other every week, perhaps every day each day taking in a new group, and have already done the small talk and are now talking to each other regularly. But don't get too jealous. When the only thing you have in common with the group is that you reproduced around the same time, the friendships may not last.
I used to just enjoy seeing my kid with new toys.
Just be bold, if it's important to you. Sit down, smile, "Wow, they really love that train thing, don't they?" or "That top is so cute!" or some other nice thing about their kid. THEIR kid. Don't sit down and say something about how amazing your kid is. That actually happens, it's hilarious, some random sitting there babbling at you about their kid's recent genius acts. Or, the opposite, don't sit down and criticise your kid - "Oh god, mine's a nightmare, never sleeps, always screaming, I am so tired and I scream at my husband all the time..." Yeah, no. Not fun.