@educatingmarmalade I would say something different, or different in some respects. You say that babies were removed but that an older nursery aged child is left in her care, and that her reading and writing skills are not great - it sounds as though the babies were removed because the mother was not coping - perhaps PND perhaps other - it doesn't sound as though she is trying to get you to play good cop or manipulate from what you have said. I think that as long as you stay aware of boundaries, and keep in mind the more stark warnings you have received in this thread, you should not change the way you do things, not steer conversations any one way or the other (unless you feel boundaries have been crossed), stay in partnership, as normal, because of the child who is with you.
Assuming you give photos to other parents too, I think it is fine to give her photos - I am not so sure about cards, but photos would be a good start for her and don't cross any lines.
I don't think offering condolences is a good idea, or raising anything with her, but in your shoes I would continue listening and reacting in a human way as well as a professional way. If she asks your opinion or for advice (and if she hasn't so far I can't see that she would) then you can very honestly say that you have no experience.
I think you need to get advice in real life about how to talk to the child about it, if it does come up, as you won't be able to steer her away if she has chosen to talk to you.
I am wondering if you are the manager, but that there is someone or an organisation you could get advice from?
There are groups out there which help people in her situation which may well provide her with more emotional support and guidance than the SWs involved, and so hopefully she has been pointed in the direction of these by someone.