I think others have covered the idea of being direct and controlling the schedule yourself.
I would supplement these approaches with some behavioral feedback to let the coworker know that you're still on friendly terms but that this subject is one not relevant to your work. In his book 'The Like Switch' Jack Schafer highlights the 'big three' friend signals:
- The eyebrow flash: a quick up-and-down of eyebrows that signals recognition.
- Genuine smile: acknowledgement of the other person via smile
- Eye Contact: acknowledgement of the other person via eye contact (1-2 seconds)
- Greet by name (optional)
When you decide to directly discuss this issue with your coworker, open with all of these behavioral signals. This communicates to the other person that you are a friend who respects and acknowledges them, meaning they are more likely to listen to your explanation.
Smile and communicate kindly and firmly why you cannot view the pictures/chat (e.g. 'I'll stop by your cubicle when I have time', 'I prefer to keep my home life and work life separate.', 'Let's talk at the end of the work day', 'I really need to concentrate on my work this month because of XYZ"). Smile genuinely again, make eye contact and thank them for stopping by. Break eye contact and usher them out/turn back to your work.
A genuine smile is important. People are very good at detecting false smiles, eye rolls etc. (Schafer); I try to think of something I genuinely like about the person and keep that in mind. An additional behavioral trick is standing-up when someone enters your cube/office and you don't have time for them. This usually does not allow them to sit down and puts you at equal height/eye level with the individual allowing you better communication and control of the office space. This does not need to be aggressive; it's the same signal we give when we stand at the end of a meeting to signal it's over.
If the coworker returns to share later, smile and refer her back to this conversation (e.g. 'Thanks for sharing Carol...remember when I said I need to concentrate on XYZ this month/remember when I said I like to keep my home and work life separate/remember when I said I'd come by later...I've got to get back to finishing this right now.') Repeat this every time she comes by for the child's photo purpose while also smiling/acknowledging her. This gently puts the social faux pas on her for returning as she did not remember your conversation from earlier.
I think people are mostly hurt when they feel their social effort is rejected. Try to communicate on all levels you still have a friendly interest in her and appreciate her social foray, but kindly and firmly outline this subject is not of interest to you personally. Best of luck.