I agree with Lemongrove on all counts.
At 15, she was a child, and not of the legal age of consent or majority. I think she was let down by the UK, who had a duty of care to stop her from being radicalised, and to prevent her from leaving the country for Syria, or wherever it was she went en route to Syria.
As others have said, we don't know what happened out there, but I'm sure it must have been grim in the extreme. She couldn't possibly have had any idea what she was getting into.
FindingNemo - her babies were born (and died) out there, not before she left, so I'm not sure what they have to do with it, other than to make it even more likely that the balance of her mind would have been impacted during pregnancy at her age, and because of losing them all - not to mention the sort of 'lifestyle' that allowed her to get pregnant so soon after giving birth then losing the previous baby. Her hormones and mental state must have been all over the place - have some compassion.
If she had been older, I would still have reservations about stripping people of passports and citizenship, but would have less human sympathy. As it is, a child is not, IMO, capable of a full understanding of the finality of their actions, particularly when she has been groomed and has undergone such a distressing experience.
Finally, even if she comes back and is given UK citizenship, she would still be tried and (probably) found guilty of some sort of terrorist offence, although I don't think it is clear what exactly she has done. In that case, she will be dealt with under UK law, which is the law that people are saying is the one she has broken. That seems like justice to me.