It is critical that you address the eating style of your toddler right away, though you also mention that you have a pre-teen whose eating habits you are concerned about. The healthy diet challenges for these two age groups are a little different, but this answer should help you address both of them together.
1. Balanced diet
Try and get sufficient proteins, carbs and minerals in your youngster's diet. The easiest way to do this is to make sure there is at least two different fruits and two different vegetables in their diet everyday. Try and get in adequate amounts of meat, fish and eggs in their diet. Unless there is a tendency towards obesity, don't cut out the good fats or the red meat, since they are essential for growth. Make sure hydration is adequate by getting both your kids to consume close to a liter of water a day. It will not be easy, but keep it as a target, so that you can keep their hydration up.
2. Getting them off junk food
Admit it, junk food is tasty. The reason for that is artificial flavoring, higher levels of sodium (salt), and frying techniques. You will have to inculcate in them an appreciation for the fine tastes that come from other ways of preparing food. Encourage them to explore different cuisines of the world by preparing them at home. Experiment with unusual ingredients and subtle flavors and explain to them what you have done. Start your junk food weaning program by slightly stepping up on your salt use, and then gradually tapering it down. Experiment with caramelization of vegetables like onion, potato, carrots, and techniques such as grilling, roasting and baking to introduce them to a new range of tastes and textures. Kids get a thrill out of sharp and stimulating tastes, so explore lime and tamarind, palm sugar and fruit preserves, mint and cilantro, fish sauce and concentrated stock, both vegetable and animal protein stock, etc.
3. Make food an adventure
Eating is a multisensory experience. Use different colors, shapes, techniques, and cuisines, to turn meal time at home into an adventure. Involve your pre-teen and your toddler in the kitchen. Male pre-teens may balk at kitchen tasks, but will take pride in participating in grilling and roasting activities.
Toddlers will take pride in washing vegetables, handing things to you as you need them, and in general being an active participant in the cooking, even if that means just standing around. Share with them your understanding of the transmission of life that takes place in the process of cooking and eating, and encourage them to respect food, and not treat in merely as an object of gratification.
