Kate Middleton knows well how to handle her 4-year-old son Prince George and 3-year-old daughter Princess Charlotte.

On Sunday, the Duchess of Cambridge was spotted with her two older kids to cheer Prince William at his charity polo match in Gloucester, England. One onlooker even praised Middleton's parenting.

"Kate is a fantastic mum," an onlooker told People. "She was calm with the children. She let them have their head."

The onlooker also observed how Middleton used a proven mom technique to handle her royal children. "She seemed to say to [George, 4, and Charlotte, 3], 'First to the polo field is the winner!'" the observer said. "She was trying to get them tired before they had to get back in the car to leave."

Middleton's children seemed to be enjoying their playdate. Princess Charlotte was seen performing forward rolls and headstands. She was also photographed running barefoot.

Meanwhile, Prince George was seen playing with a toy gun with a friend. He was also photographed entertaining himself with a rainbow slinky.

Although Prince George and Princess Charlotte's charm stole the public's attention, many also noticed Middleton's weight loss. It's only over a month since the duchess delivered her youngest baby, Prince Louis, but she seemed to be back at her pre-baby figure.

"How does Kate Middleton basically lose her baby weight immediately after giving birth and I'll be six weeks into my summer fitness routine, still trying to lose my 'I ate too many snacks whilst revising' weight," one netizen asked.

According to Laura Spicer of The Women's Health Clinic, Middleton's post-baby body could be due to her genes or a healthy pregnancy. There are moms who can easily get back into shape.

"Some women definitely can snap back into shape," Spicer said. "However, if you are fit and have had a healthy pregnancy with a normal BMI it is possible to be similar to your pre-pregnancy body shape. It's also got a lot to do with genetics, so if Mum recovered well you’re more likely to as well."

Middleton is still on maternity leave. The duchess is expected to return to her duties in October.

Kate Middleton Kate Middleton used this mom hack at Prince George and Princess Charlotte during their Polo playdate. Pictured: Middleton, Princess Charlotte, Savannah Phillips, Prince George, Prince William watch the flypast on the balcony of Buckingham Palace during Trooping The Colour on June 9, 2018 in London. Photo: Getty Images/Chris Jackson