"Kate will be a firm but fair mother," the 'Us Weekly' magazine quoted her friend who is supposed to raise her baby-to-be in Apartment 1A of Kensington Palace.
Among the 30-year-old Duchess's top parenting priorities will be to keep her child grounded despite the little one's privileged upbringing, the magazine said.
"Kate will let her children have fun and freedom, but she will also make sure they do what is best for them," a longtime pal told the weekly.
According to the magazine, the child - third in line to the throne - will jump ahead of William's kid brother, Harry, in line for the throne now that the UK has enacted a constitutional change that gives a firstborn girl the right to succeed the throne over her younger brother.
Middleton will be the first commoner in 350 years to give birth to the future king or queen of England, the report said.
St James's Palace announced Kate's pregnancy, saying that the Duchess has a severe form of morning sickness and is currently in a London hospital. William was at her side.
The news drew congratulations from around the world, with the hashtag "royalbaby" trending globally on Twitter.
