You can't accept food or drinks, unless it's at an official function.
When the royals take part in a walkabout — or stroll — with a crowd of people, they're not allowed to take food or drinks from any folks they meet from that crowd. Why? For security reasons. Back in the day, their ancestors used to use royal taste testers. It does sort of make sense why they won't accept food; there's no way to verify that it's safe. However, when royal family members attend official functions, they are allowed to eat the food.