1. Each family provides food for solely their child. Nanny makes sure that each child eats only their food. This is most common when there's an allergy (or possibility of an allergy) and/or major dietary difference (one family is vegetarian).
2. Nanny shops for and makes all food for the share, parents provide equal amounts of money either up front as petty cash or loaded on a card, or nanny volunteers to be reimbursed. This is easiest for the parents. Some nannies wouldn't want to take two infants or toddlers shopping, while others would have fun with the kids while doing it.
3. Parents do the share shopping from nanny's list, and nanny makes all share food. This is easier for the nanny than taking kids shopping, and parents don't have to worry about whether the nanny has enough money for the food. Parents can switch weeks providing food, one set can reimburse half of the total, some families buy four weeks then switch. There's tons of ways to work it out.
4. Parents shop and make all share food, nanny is only responsible for heating it up. This is very uncommon.