Cooking for my kids (3yo/5yo)

Since you're already on the "I'm not a short order cook" train, you don't need advice in that direction.

So you do what my husband and I do. We have different calorie and macro needs, but we eat the same foods every night at dinner. We just both eat different portion sizes of said food. Sometimes he'll add some bread to his, or I'll sprinkle on some cheese on mine to account for different macro needs, but the base meal is the same.

We also snack on different things to fill in those gaps.

Make your meal, weigh out your portion and then serve them the way you would have before. Keep in mind that you don't always have to cook "healthy" meals. Teaching healthy eating is important, but you don't have to completely cut out things like mac & cheese or pizza. Just work those into your calories/macros once in a while. Losing weight does not mean you have to eat boring food.

My toddler has more calorie dense foods than I do, in addition to our meals. He is tall (99th percentile) but has always been slimmer - currently 28th percentile for infant BMI. So I understand your thinking about wanting to make sure they eat enough. I have go-to items that are calorie dense that can be added to the same meal or as a snack, eg: peanut butter (we use 100%, no added anything), full fat natural yoghurt to which I usually add fruit puree, cheese (usually melted mozzarella!), avocado pieces, hummus, malted fruit toast. He is a fan of bananas, raisins and rice cakes as snacks in particular. He has a large glass of full fat milk at breakfast every day, to which I usually add a milk mix. All of these different things seem to keep him going and he has never been underweight. As my husband's grandma says, "if he's looking skinny, add butter"