Hi,
I agree with Mark about the insurance issue, which you need to check out. However I'm not sure about the 'communicable diseases' part. Although babies and very young children do appear to spread things very quickly, I think the media has a tendancy to scare people around the swine flu situation. Still, you should get some professional advice and take any necessary precautions. Policies and procedures should also be developed to ensure everyone understands how your creche will work once it's up and running. Perhaps you could ask your staff to give their views on the subject.
Personally I think a creche is a good idea, and should form part of your work/life balance strategy. You should also promote a family friendly organisational culture.
Hope this helps.
Cheers
Michaela