This depends on what you and the helper agree on during the interview and what your household situation is.
In most cases, 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 significantly 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.