Zara, the daughter of Princess Anne, was joined by Harry as Prince William competed on horseback at the Festival of Polo, at Beaufort.
Mia recently drew the spotlight herself thanks to an adorable appearance in one her great-grandmother's birthday portraits.
"The Queen got it quite right when she told her, 'Just hold the handbag now,' and she did that," dad Mike Tindall told Good Morning Britain. "It ended up being a great photo of her.
"It just turned out to be the right photo at the right time and it will be something we'll always cherish, I think," he said.