I totally understand where you're coming from - buying healthy, whole foods can sometimes be more expensive if you don't already have some staples in your kitchen. And cooking for a large family is already tough as it is. But I'm here to tell you that everything is going to be alright!
What I suggest you do is read the entire Part 1 of the Body Fuel System and really soak up all the information. You can use the knowledge you gain to make this system work for you. You absolutely do NOT have to follow it exactly.
A good idea for you is to also read through the recipes for each week and pick a couple that sounds good to you and that your kids will enjoy. Test them out. Use the guidelines in Part 1 to pick your other foods and meals. You don't have to buy exactly every item as listed in the grocery lists. You can adapt the system to suit your budget, lifestyle, and food preferences.
If you can't buy all organic, don't sweat it. Although it's ideal, the important thing is to buy FRESH whole foods and start eating a diet full of nutrients. You don't have to buy different types of flours and nuts if you can't afford that much variety. Just stick to one or two staples and make it work for you. If a recipe contains an item that you can't splurge on (like hemps seeds), substitute a nut with healthy fats like walnuts.
This lifestyle should never be about stress. I encourage you to do the best you can with what you have and not let the rest get to you :) You are making great healthy choices for you and your family and you should celebrate that! You should still be super excited about the program and learning a whole lot about making healthy food decisions!!