Is it cost effective to buy all organic foods for my child?

No. It is not necessary to buy all organic food. There are certain fruits and vegetables that have a high pesticide content (go to www.Ewg.Org to see list). These should be bought organic. Meats should be clean (free range, grass fed, hormone free). Eggs should be free range and fertile.
Waste of money. Recent studies have shown that organic foods have shown no nutritional benefit over those grown or developed through regular methods.If you like the idea, good for you, but the hoped for benefits have not been validated.