Marinate beef - 30 minutes for ground beef, at least 3 hours for thinly sliced beef
Cut the veggies as thinly as possible to make it easy and safe for young children. I love to use a mandolin for this as it cuts down the prep time significantly!
In a small bowl, combine all the sauce ingredients. Set aside.
Add 1 teaspoon sesame oil to a non-stick pan. Swirl to coat. Cook the onion over medium heat until it softens, 1-2 minutes. Transfer to a large mixing bowl.
Add 1 teaspoon sesame oil to the pan and cook the carrots over medium high heat for about 1-2 minutes. If cooking for baby, add a tablespoon or so of water and continue cooking until they get nice and soft, about 3 minutes. Transfer to the same bowl with the onion.
Add 1 teaspoon sesame oil to the pan and cook the bell peppers over medium heat for about 3-4 minutes, until softened. Depending on how thinly sliced they were, you may or may not have to add water as suggested with carrots. Transfer to the mixing bowl.
To the pan, add the marinated meat (no oil necessary) and cook over medium heat until it's no longer pick, about 3 minutes. Transfer to the mixing bowl. Keep the beef juice in the pan!
Cook the noodles. Bring a pot of water to a boil, add the noodles and cook for 6-8 minutes. Rinse in cold water and drain. Cut the noodles with kitchen scissors into short strands.
Reheat the meat juice in the pan. Add the noodles to the pan and cook over medium heat, stirring frequently, until translucent and sticky, 2-3 minutes.
Transfer to the large mixing bowl with all the rest of the ingredients. Toss everything together with hand. Set aside a portion for the baby. To the rest, add the sauce. Adjust seasoning to your taste. You can always add more soy sauce and sesame oil.