It depends on what is agreed between yourself and the helper during the interview, and if there are other influencing factors such as your household dietary preferences.
Most of the time, no food allowance is needed. The helper would be able to access your fridge and/or pantry and add some items for herself when she buys groceries. Feel free to save a shelf only for your family if you have items you do not wish for her to use.
If the employer’s household has different dietary preferences from the helper, a food allowance is useful. It is usually $40-$50 a week and the helper will exclusively buy her own food.