I think it's more important to consider the quality of the food/calories rather than quantity. There is a big difference between eating 450 calories worth of candy and soda and 450 calories of processed frozen meals and 450 calories worth of fresh, whole foods. I think the Dr. Sears book probably has caloric guidelines for children. Also Dr. Sears really likes smoothies for kids. I've been making myself nutritional smoothies in the mornings because I don't have time to get all that health stuff packed into a compact meal in other ways, and it's been great. It was hard to get in the habit at first, but now it's just automatic. But, basically I believe the basic guideline for average women is 1500/day. You can scale that down for your kids, but then add in a bit because they have higher metabolism and are growing.
We try to stick with the last option as much as possible, which surprisingly does not really take that much time to prepare (pop some chicken in the oven, rice in the cooker, steam some veggies), we resort to the middle option, like mac-n-cheese or just pasta with sauce and peas when we don't have the other on hand (or 50 minutes for the chicken to bake), and we do not have any candy or soda.
If you just have healthy foods around, you won't have to worry about the calories. I think cheese is great because it's filling, doesn't hurt their teeth, and is nutritious. Personally I have a chocolate weakness, but I strictly keep it in the closet- at work! So my son never sees it.
Best wishes!