And expect fun and fireworks from the playful prince.
"He will be a wonderful uncle – [think of] the mischief he will want to get up to with the [children]!" a friend tells PEOPLE. "I can see it already."
That would match his behavior at that other great occasion in William and Kate's life – their wedding. It was Harry, 28, who had the young flower girls scurrying around his feet at the Palace reception, a guest told PEOPLE at the time.
Harry, who is serving as an Apache attack helicopter pilot on a tour of Afghanistan until early next year, has had to make do with phone calls to and from his brother and sister-in-law since they broke the happy news of their pregnancy.
And he is said to have sent Kate, who is recuperating at home following her attack of severe morning sickness, a get-well message from the frontline.