It will save you much aggravation, and it will certainly make your life together more harmonious and pleasant. If a change has to be made in your schedule, do it on paper so that it is clear. Preparation gets you off to a good start. Discuss what she can bring. There are, in some states, very strict regulations about this.
Schools usually send home information and a sample lunch suggestion.
If your child has allergies or other food restrictions, decide how you are going to handle the lunch situation. Tell the child that his lunch will be different and why. I find that children who are prepared do not mind at all to be different. In general, children are very understanding. Very often you will see a friend explain to another why Johnny cannot eat this or that.