Apart from the fact that this sweets ordeal can be prevented almost entirely, if you're early adopting a cautious and disciplined policy, my best advice would be to tell them the actual truth:
Because sweets are poison
If above doesn't 'take', there's further elaboration …
It makes people die
Effects of sugar
'Poison' may not be the perfect epithet for something that has deleterious effects on health. Likewise, the 'makes people dead' statement obviously isn't meant to imply that people are dying on the spot by eating candy – rather, that candy has long-term effects which by extension may lead to life-shortening or lethal outcomes (more or less likely depending on the amount of eating, of course).
I think common repudiatory arguments such as, there's lots of other things which potentially risk making people ill/unhealthy/dead – other kinds of consumption articles, living circumstances, stress behaviors, etc. – fails to answer the question regarding candy: why worsen the odds of a healthy life for your kids?
I'm not a scientist by any stretch – just a layman and parent who cares a lot about my two sons. Therefore, I'd like to add a disclaimer that there might be flaws in my interpretation of the things I refer to below. Now:
Among government and institution reports, scattered articles etc., I base my statements in the answer (where it can be argued that I inappropriately stated that it was the 'actual truth') on the research of one of the world's most reputable specialists on pediatric hormone disorders and leading experts on childhood obesity, Robert H. Lustig. According to him, some of the prominent long-term consequences (or at least risks) of added sugar – that is, sucrose (refined sugar) or High Fructose Corn Syrup (HFCS) are:
- fatty liver,
- insulin resistance,
- heart disease,
- cancer …
Maybe that's enough bad stuff to warrant calling it poison? I don't know, it's my personal view.
Further info on the predicaments of sugar according to Lustig (which I find very informative):
"Is Sugar Toxic?" (NY Times article, 2011)
"Sugar: The Bitter Truth" (Robert H. Lustig, 2009 seminar; YouTube)
^Same seminar, March 24, 2011; YouTube
Poison in candy
Apart from the deleterious effects of added sugar in sweets, there's also perennial reports of candy being recalled. A few sources related to that:
Toxic Treats Index: The CA state Childhood Lead Poisoning Branch, Food and Drug Administration and Orange County have conducted tests for lead in candy since 1993. Out of 1,503 individual test results, there were 400 tests showing lead-contaminated candy – 261 for candy samples and 139 for wrappers.)
A list of the most recent withdrawn candy
CDPH (California Department of Public Health) - List of recent test results for lead in candy
CDPH - Photos of candies found to contain lead – updated December 29, 2011
Lead poisoning from candies – Centers for Disease Control and Prevention (Seems mostly pointing at mexican candy imports.)
"Toxic Waste Nuclear Sludge" recalled over high levels of lead (A pretty recent event.)
"Minute traces" of stainless steel in Colombina Mega Pops Lollipops (Yummy.)
Formalin-glazed Candies, Anyone?
Thallium poisoning from maliciously contaminated marzipan candy
The list could go on indefinitely.
… There's also a bunch of candy containing allergens (unlisted sulfites, peanut traces, etc) but I guess that's peanuts (pun) compared to heavy metals and the long-term effects of added sugar. Again, candy shouldn't be singled out as a unique example of recalled consumption articles, obviously, but the point I'm trying to make is, stay alert to what kind of candies you let your kids eat, if any. Contemplate the amount of added sugar your kids consume. Without laying any further weight to whether it's justified or not – well, I fail miserably hiding my personal bias – at least ponder Lustig's notion of thinking of sugar, like cigarettes and alcohol, as something that's killing us.