If you look on David Little's website,
www.similibus.com
(I think). You will be able to find some remedies to keep handy for malaria. Natrum Muriaticum and China are 2 that come to mind, right away.
Drosera may have been used as a prophylactic in a specific epidemic IF it was determined to be one of the Genus Epidemicus remedies for that particular outbreak.
This does not mean it should be used as a prophylactic. It can be a good remedy to have on hand should the whooping cough actually affect the child.
The best thing to keep on hand in the case of any approaching illness is the first aid kit, and, if you're lucky and see a homeopath already, you may know of your own similimum remedy. That is likely to address any illness, should it befall the child. In any case, nothing works better than giving the appropriate remedy to match the symptoms.
Other than that, the best thing to do is wait until you have symptoms on which to prescribe before giving a remedy.
Best precautions are still eating well, maintaining good hygiene, getting enough rest, and drinking enough fluids.
Also consider the well documented fact that those who seem to be the most susceptible in these whooping cough outbreaks are children who have been vaccinated against the disease. It may very well be that this unvaccinated child will have all the resistance he needs simply because he was not vaccinated. Let's hope so.
Divina