You have no idea what risk she poses. What she would like to do (she thought the Manchester bombings were justified), what she's capable of doing (she was intelligent and resourceful enough to get to Syria as a child, despite being on the state's radar), what training, contacts or resources she gained in Syria - we only have her word for it het activities were confined to those of a housewife; she might be an expert bombmaker.
Admittedly, nor do I know the risk, but I'm less willing to give her the benefit of the doubt.
The maximum for that offence is ten years. That'd be for the committed adult male fighters who tried to recruit others. She'd say she was a child, was brainwashed, never fought, never recruited. There'd be psych reports saying she has PTSD etc., and reports about the impact of prison on her child. She'd probably get 4-6 years, of which she'd have to serve half, some of which would be out on a tag - she could do not much more than 18 months.
Effective surveillance of someone who knows they're being watched is near impossible, and would be relatively easy to avoid. The example I have above of an unregistered mobile. Or, if, say, she leaves home with a suitcase and goes to the tube, then what? She gonna be stopped every time, just because someone is watching her house. What if she drives out of her garage with a bit full of explosives? Pull her over every time she uses a car?