I work in a special ed preschool at a public school and our kids bring their own snacks and lunches. Sometimes, a parent wants to bring something for a birthday party. The only thing we ask is that the portions be individual or something we can divide up easily. For example, if they bring a fruit tray with yogurt dip, each child has to have their own portion of the dip - which means the parent needs to bring bowls or plates. We don't allow the kids to take food from a platter themselves or dip into a communal dip bowl. We have to be able to divide it up before they eat it so they don't touch each others food. We do this for health reasons. If they bring a box of crackers, we can easily pour those on paper towels so everyone has a portion. Since we're a public school, we're not provided with things like paper plates and bowls. If we want those in our classroom, the teachers have to pay for them so we generally don't have a large supply. Same with plastic forks and spoons. Most private preschool do tend to supply those things.
I would say whatever you bring, make sure you also bring whatever is required to serve it properly.