What should you do if a child drinks hummingbird nectar?

Give him singing lessons. No. Seriously. When a child drinks or eats anything suspicious, call Poison Control, and the number should be easy enough to find in your directory. If the child is diabetic, it is more urgent to call. Have the package handy so you can review contents. There is an 800 number in my directory: 1-800-222-1222. We usually don't allow telephone numbers to be put into answers, and this may be edited, but under the circumstances maybe this is allowable, at least for a short period.

You should call poison control, but off hand my opinion is that not much harm could come to the child, unless a large amount was consumed, or if the child is diabetic. Humming birds require a lot of energy to keep up their jaunty metabolisms, and so the nectar will be very high in sugars.