Stephanie Wallis, the founder of Safe and Sound Event Childcare spoke exclusively to Express.co.uk to explain.
She said: “Young children enjoy nothing more than celebrating their parent’s birthday, they love to plan what they are going to buy and make for them and especially love helping to open the presents.
“As Prince William’s 40th birthday lands so close to Father’s Day (which we have just celebrated here in the UK) it’s highly likely the family would have had a joint day tying both of these celebrations together in a bigger way this weekend with an extra little celebration and moments together on Prince William’s birthday.
“We saw a precious photo which was released on Father’s Day of Prince William with his children on holiday in Jordan which William posted on Twitter in order to wish a Happy Father’s Day to the world’s fathers and grandfathers.
“With this being William’s 40th birthday, a milestone birthday, I predict he will take some time today with his children to reflect back on his years and what he’s achieved.
“There is nothing more precious on a milestone birthday if you have children to sit with your children on your knee, get the photos out and recall fond memories of their childhood with you so far,” the expert noted.
How has William celebrated his birthday in the past?
Ms Wallis added: “Last year the family all attended the half-marathon Father’s Day Race at Sandringham.
“It’s a lovely experience to sometimes share part of the day with other families.
“I’m sure Prince William will wake up to breakfast in bed and homemade cards and maybe some chocolate biscuit cake, one of William’s favourites!”
Prince William and Kate Middleton’s children attend Thomas’s School in Battersea, which is where they’ll be during the day.
Ms Wallis commented: “As William’s birthday lands on a school day the family might save the special tea and cakes they have made until the evening.
“No doubt they will blow up balloons, string up decorations and there may be even some party games planned.
“Apparently the two popular board games in the family household are Monopoly and Risk.
“The excitement your children have for your birthday is contagious.
“You can’t help but be a little excited, it is a truly special time and creates such treasured memories.”