The 35-year-old took to Twitter when news of her ex-husband's new arrival broke on Wednesday, writing:
'Yaay I have the kids till next week x'.
And Katie, who also gave birth to baby Jett only 5 months ago, made the most of family time yesterday.
The model, who usually favours a more 'made-up' look, was spotted looking surprisingly toned down and natural.
Leaving husband Kieran Hayler at home, perhaps to spend boys time with her eldest sons Harvey, 11, and 8-year-old Junior, Katie spent the day with her two youngest children.
The book author took 6-year-old Princess and baby Jett for a spot of shopping in Mayfair, before heading to a posh eatery.
Princess and Junior are thought to be extremely excited, as they should be meeting their new sister very soon.
This week Pete announced girlfriend Emily Macdonagh had given birth, writing on his site:
'We are extremely thrilled and mum and bubba are doing fine.
'We are looking forward to a nice quiet couple of weeks getting to know our baby. Thanks to everyone at Musgrove Park Hospital.'