This depends partly on what type of allergy your child has. Some allergens --- nut and shellfish allergies, for example, and most fruits and vegetables --- are fairly easy to work around in most recipes, and to avoid on labels. Other than being extremely careful when reading labels and avoiding allergens in the produce aisle or at the seafood counter, they may not affect your day-to-day life much while cooking at home.
Wheat, soy, corn, and dairy allergies are probably the four most problematic for home cooking. All are present in a staggering variety of prepared foods and are base ingredients to a wide range of foods. For these allergies, you will find it worth your while to learn to substitute and to find good allergy-friendly alternatives to common foods. You may also want to buy specially prepared allergy-friendly foods, or invest in an allergy cookbook. Allergy-friendly foods are available not only at health food stores but at a growing variety of supermarkets (usually in the "natural foods" area) and online.