Katie wore a bubblegum pink wig for the event while Kieran dressed as her handsome prince in a satin blue cloak - and a frog mask.
The couple posed for pictures while kissing, before Kieran removed his green mask and puckered up for some more smooches.
Former glamour model Katie has previously said that she had to "kiss a few frogs" before finding her prince in third husband Kieran, so we're guessing that was the thought behind the costumes!
The couple, who are expecting their first child together, appeared in London to launch her new novel He's The One. The book is about an actress called Liberty Evans who is longing for the man she loved before she became famous.
Katie's decision to cover up her growing tummy in this elaborate princess costume appears to have been a conscious decision.
When recently asked by a fan why there haven't been any pictures of her with a bump, Katie replied: "I know but trust me I have a bump lol.
"I've chosen not to make money out of being pregnant and kept it hidden xx"