I like the idea of putting larger and larger objects in their pussy (and subsequently womb) until they're torn apart from inside. Maybe have a point where their womb is torn and the insertions are squishing their internal organs into pulp, but they still continue because they think they can get an even larger insertion, so why stop here. A fire extinguisher surely must be involved on the way.

Also the beheading idea reminds me of one of mine, where Len and Rin wonder what it feels like to be beheadead, so they behead Rin and Len looks into her eyes to see what she feels, and physically she feels immense pain that makes her only wish to die sooner so it would end, but the pain makes it feel like it lasts forever, but at the same time neither Len nor Rin herself regret anything, it doesn't even occur for them to regret it, they were curious what it would feel like and now they know, so their curiosity is properly satisfied, and there's nothing wrong with experiencing a pain so overwhelming it just makes you want to die as soon as possible, if that's what beheading ended up feeling like, then cool, now they know it.

As for girl twins, they might shove their heads into each other's wombs and suffocate. Maybe it's some sort of unbirth play, or maybe they just find it funny. Like they're casually 69ing and then one out of nowhere comes up with an idea to shove her head in her sister's womb. The other sister treats it maybe kinda like being tickled or maybe like some playful wrestling? Like she kinda minds it but not really, actually she finds it funny too, so ends up shoving her own head into her sister's womb too. Maybe there's some like not really serious fighting involved, where each sister tried to not let the other get into her womb, but not really, just having fun. So by the time the second gets inside the first one's womb the first one is already suffocated from all the fighting while inside her sister's womb. So then the second one goes "huh, you're already dead? oh well, all the fighting mane me kinda tired too" and relaxes, suffocating too.

A combination of the unbirth and beheading ideas is: one girl tries to get inside the other's womb, playing like she's her baby, but she can only get her head inside and laments about it, so the other girl decides to help and cuts her head off, this way if she's only a head, she's all inside like she wanted.

Another idea is the kids find something that is shaped like a dildo, but clearly not designed to go into one's pussy. Like a cylindrical grater. Before I used it in my Suspicious Silence story, I had an idea where Len and Rin see it like it a store and laugh about it like "lol that grater looks like a dildo" "haha it would tear my pussy to shreds" and proceed to actually trying it just for the laugh. But since I've already used the grater in the Suspicious Silence story, maybe you can come up with something else to the same effect. But I also wouldn't mind if you just use the grater.

A girl might impale herself on some pole, like maybe a sidewalk pole, but she isn't really trying to impale herself, but to see if she can perform a side split with that pole in her pussy, and in the end she can, although the pole ends up tearing her from inside and maybe exiting through her belly, maybe through her chest, missing the lungs and heart so she has time to admire her own gymnastic ability before she dies, maybe through her mouth, depending on how long it was to begin with. Anyway the girl is satisfied that she successfully performed a split and doesn't bother to acknowledge the fact she ended up impaling herself all the way through, like its totally irrelevant.
Oh, since we have two girls, it might start with one impaling herself just slightly on that pole to masturbate, then somehow the splits end up mentioned, and she just doesn't bother to get off the pole before giving a demonstration. The other girl can then help her sister by pushing her down on her shoulders.
Then instead of her dying it can end up with her just getting up (probably with the help of her sister) and they just go on their way, but the impaled girl has a gaping pussy and optionally a cervical prolapse, through which blood and pulped internal organs leak out, but they just go away not bothering to acknowledge that.