lily DD is 3.5 and I think quite naturally low carb I think, although this is difficult with 'kids' food. We try to eat dinner with her as much as possible so her evening meal for the most part is very low carb - meat, fish, vegetables. She has yogurts if she wants dessert, but only if she is still hungry not as a matter of course, or berries.
Breakfast is usually the biggest carb bit of the day - porridge made with full fat milk, two boiled eggs and one slice of toast, or toast with peanut butter and jam, or bacon and eggs!
Lunch is hardest as at the childminders she just has whatever she is given, but if at home, I have salad with meat or cheese so she tends to have meats, cucumber, tomato, maybe a carrot and picky bits, if she asks for bread or sandwich that is fine with me as well. Snacks consist of cheese cut into cubes, seeds (sesame and sunflower), or nuts (pine nuts or other ones chopped up), or pepper, cucumber, carrot sticks. Sometimes fruit or oat bars etc...
She does of course have pasta sometimes and pizza or rice and couscous or jam sandwiches and crisps, but I think at this age I am fairly happy with the range of what she eats and she is brilliant at knowing when she is full and when to stop eating things. I have tried to never not let her est things, or make food an issue. i have never used it as a reward or made anything a 'treat'. For example if she has chocolate then she will often have a few bits, then ask to save the rest for another day when she is full. I think I am very lucky with her tbh, and have no doubt that in a few years she'll be the fussiest eater on the planet!!!!!
Sorry that has turned into a bit of a essay, but my main advice would be to eat with them as much as possible!