Although the Duchess of Cambridge was expected to join her son for the important milestone in Sept. 2017, she was forced to cancel her plans

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Kate Middleton was all smiles as she and Prince William accompanied their children, Princess Charlotte and Prince George, to school on Thursday.

The milestone marks 4-year-old Charlotte’s first day at Thomas’s Battersea — where big brother George, 6, also attend. Charlotte was happy to have her mom on hand for the big moment. The typically “outgoing” princess was noticeably shy on her first day of school. Charlotte clutched mom Kate’s hand and nervously played with her ponytail on their walk to the school’s entrance.

Just two years ago, Kate was unable to walk George to his first day at the $23,000-per-year private school some three miles from the family’s Kensington Palace home. Although Kate was expected to join her son for the important milestone in Sept. 2017, she was forced to cancel her plans due to acute morning sickness she was suffering during her pregnancy with Prince Louis. Hyperemesis Gravidarum also afflicted Kate during her previous two pregnancies.

Princess Charlotte to begin school
Princess Charlotte, Kate Middleton, Prince George and Prince William
| Credit: Press Association via AP Images

“Unfortunately The Duchess of Cambridge remains unwell, and will not be able to accompany Prince George on his first day of school. The Duke of Cambridge will drop off Prince George this morning as planned,” a spokesperson said in a statement at the time.

Princess Charlotte
Kate Middleton and Princess Charlotte
| Credit: Aaron Chown - WPA Pool/Getty Images

Prince George flashed a big smile at home while holding hands with his father for his official first day of school portrait.

Prince George Attends Thomas's Battersea On His First Day At School
Prince William and Prince George in Sept. 2017
| Credit: Chris Jackson/Getty

But nerves appeared to take over as the uniform-clad prince made his way into the school, shyly shaking hands with the head of Thomas’s lower school, Helen Haslem.

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Prince William and Prince George in Sept. 2017
| Credit: Richard Pohle/The Times/PA Wire

A source previously told PEOPLE that the future monarch is “very happy” at his school, where he is simply known as George Cambridge.

Princess Charlotte may also experience first-day jitters, but sources say the outgoing royal (who previously attended Willcocks Nursery School) is thrilled.

“She can’t wait to be with George at big school,” an insider previously told PEOPLE. “She is so excited about it all.”