If you join a terrorist organisation, that's aim is ultimately the destruction of your own, I think the government has every right to make this as difficult as possible, even to go as far as to strip of your citizenship.
However in the UK, we believe people under a certain age cannot be held to the same way an adult is. She was and probably is still, heavily influenced by those that sort to abuse her, to coerce her into joining a cause she had little real understand of. So how can we hold her to the same account as an adult, when we make allowances for crimes, based on age and other circumstances already?
To her, I think we should bring her back. Her citizenship should be restored, but she should be forced to undertake psychiatric care and as our laws say it, be released only at her majesties pleasure. For those unaware, a plea of mental health being accepted, could mean you stay in an asylum for the rest of your life, only if your doctors say you are ready, you can be released.
As to the baby. This is tragic. The loss of a child is awful, but if the child had lived, I think the child at the very least, should have been given citizenship.