What should conference organizers understand about childcare? The NSF now allows their funding to be used to pay for childcare expenses for scientists attending conferences in the United States. I take it that this means that organizers of NSF-sponsored conferences can offer to reimburse the childcare costs of attendees.

I periodically help to run conferences which are funded by the NSF, and I would like to offer funding for childcare to help parents attend the conferences I run. What should I know about childcare? 

From what I have read on the internet, it seems that parents would like conference organizers to make all the arrangements in advance. However, as a nonparent I question my ability to do this effectively. I know nothing about daycare. Moreover, I would somewhat prefer to make sure that childcare will actually be required before I take the time to arrange it. 

Is it enough to advertise "Funding for childcare will be available; if you require childcare, please contact us by [date] so we can make arrangements", to encourage parents to use the internet/telephone to find a provider they like, and to reimburse for the bills? Or is it important to go beyond this in some way?
I believe thinking about, and preferrably getting data on, the ages of children who would be likely to accompany their parents, would help you answering the question. An older child (say, 7 and older) has vastly different needs from a toddler.

Speaking from experience, a short-term daycare solution for children under 3-4 is likely to cause more frustration for both kids and parents than it solves. I would strongly recommend against offering this.

Another question you might like to consider is whether people travelling with children are likely to also bring their partner. I would guess "yes". If so, perhaps you could use the childcare money to somehow compensate the partner for (presumably) taking unpaid leave from work? This would be a perfect solution for infants/toddlers, and especially if a mother who is still breastfeeding (or has a small child with very strong maternal attachment) wants to attend.

I guess you could offer these as complimentary solutions: centralised daycare/playgroup system for children above (e.g) 4, compensating partner for travelling along for children under 4 (or with special needs).