What should conference organizers understand about childcare?
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).