Just in case you're only in the mood to read one sentence: the short answer is "because memories".
Soooo many of my childhood memories have been formed around really good food. A ton of our traditions would be almost unrecognizable without food, trick or treating for example. In order to enjoy these food-based traditions rather than anticipate them with anxiety and a bag of tricks to "survive holiday eating", a healthy relationship with food is completely necessary.
As a dietitian, especially as an RD working in school setting like I always have, there's an expectation for me to totally villianize candy and other foods seen as bad by society. The need to label foods as good and bad starts young. During nutrition presentations to pre-k and kindergarten classes, students often raised their hands and told me with pride that they never eat ice cream because it's bad. They also sometimes raised their hands and forgot what they were going to say, then panic and tell me that their dad has glasses instead.
I have been partly to blame for this societal fear of certain foods. I've done radio interviews discussing creative ideas for trading in Halloween candy for stickers and toys. I've developed checklists of the "healthiest" candy (healthiest in this instance meaning lowest in calories....healthy DOES NOT mean low calorie - not proud of that article). I've shared "healthier" ideas to satisfy a candy craving (nothing is truly going to satisfy a Butterfinger craving other than a Butterfinger). I've done a TV segment where I drew a ghost face on a banana and pretended to be certain that kids would find this just as satisfactory as a candy bar. All of these ideas represent a fear of food that I totally (and luckily) do not share.
I was basically being a big ole liar. I LOVE mini Halloween candy bars, I sure as hell wouldn't want to trade them in for a clementine (although I love clementines too). I know from experience and from research that trying to squash a Butterfinger craving with an apple is going to lead us to find more and more options to fight that craving until we have eaten way more than we would have had we just had the candy.
I'm not planning on showing Ruthie that it's fine to eat an entire bag of Snickers, but I want her to know that Snickers aren't forbidden. I want to help her create Halloween memories like I was lucky enough to have. Since she is only 2, she will require a little help storing and dispensing her candy (she might even completely forget about it after a day or two). But helping kids understand at a young age that candy and treats are a part of a healthy life is vitally important to helping them have a healthy relationship with candy and treats later. There are no forbidden foods. There are no foods that will suddenly take us from great health to poor health. There are foods of lower nutritional value that need to be experienced regularly in order to lose their aura of mystery and allure that can lead to a cycle of restriction/overeating as adults.
I want Ruthie to be able to sit down with a donut and eat until she is done. Sometimes she might take just a bite and leave the rest, sometimes she might lick the frosting off, sometimes she might eat the whole thing and ask for another. I want her to think of a donut simply as food, delicious food, but in no way forbidden or bad or "limited time only" or as a bribe used to eat something green.
Halloween to two year old Ruthie isn't as candy centered as it might be when she's older, but I want to help her understand now that candy isn't evil, in fact, it's a pretty fun part of being a kid.