For a pre-adolescent child, what strategies can I use to control the quantity of sweets and junk food that make it into his diet?
My first answer comes from the addiction treatment, and says this: Don't bring the enemy into your home. That is to say, don't have sweets and junk in the house, or buy them for the children when outside. Opportunity is necessary for any crime.
The second answer goes to motive. If the child is full and satisfied, she'll eat less junk. A nutritionist I know has this advice for anyone who comes to him: Don't try to remove bad food, instead add good food. If you eat healthy, nutritious meals with a good balance of fibre, carbs and nutrients, your body won't require extra sugar, so you won't crave sweets. This is true for adults as well as children.
It's very difficult to have sweets after a good meal, though it's quite easy after you've over-eaten.
Unless there is an associated health issue, don't try to restrict them completely. Try to teach them that eating a candy bar, drinking a soda, having a piece of cake, or eating a donut is okay. The key is to teach them that these types of food should only be eaten once in a while, a treat is not an everyday event.
As a household restrict sweets to a once a week special event. We have a fancy dessert every Sunday evening with our supper. We try to not eat sweets any other time during the week.
Update to clear up misunderstanding in comments:
Sunday desserts: We do NOT have high sugar desserts every Sunday night. Instead here is the list of a few of our favorites.
Yes we have routinized Sunday dessert in our house, I consider it a family tradition. It is something to look forward to each week. In reality the sugar mostly comes from cooked down fruit and a bit of honey.
If someone in our house is having a birthday then we have cake. Usually it is a boxed cake and for frosting we use sugar free Cool Whip mixed with a box of pudding.
We do not restrict them from all sugar but we keep most processed foods with added sugar out of our house. Everyone looks forward to our Sunday desserts and tries to come up with ideas. The challenge then is to try to find a healthier version that tastes good and is still considered a treat.
When we are not at home we teach moderation, and when someone (Grandpa) offers them candy we are teaching them to have a bit and turn down the rest or save it for later.
We do not count calories in our house. Instead we mostly eat foods that are healthy, with the occasional pizza and wings thrown in.
Besides "don't bring the enemy into your home", which is an excellent tip, when you do treat them, buy them reasonable quantity, out-of-home sweets. For example, you might buy each one donut on the way back from somewhere, not a dozen.
Although this is more expensive, that's kind of the point. Don't buy sweets often, and you won't waste a lot of money on them (see them this way).
I really think the whole "buy more, it's cheaper!!" that stores/donut shops do totally needs to be ignored when it comes to sweets. Do not go for bulk, discount bargains for sweets. Just buy enough to make them happy, and don't buy more just because it's cheaper per unit or seems like a good deal.
One important thing is to model good eating behavior as the parent. If you're constantly snacking on junk and gorging yourself at meals, your children will learn the same behavior.
Make sure that other caretakers are on board as well, or else they'll undermine your efforts.