The Daily Mail reports that the she took to Instagram to tell her followers about the gesture by her man's youngest child that left her feeling very choked up.
'When Fred gives me a card in the restaurant and I burst into tears,' Kate wrote on the social networking site this weekend.
The colourful card she was handed by the teenager had the words 'mum in a million' etched across the front.
Kate continued by saying that she extremely touched by the 13-year-old's sentiment.
'What an unbelievable gesture on a day that is unimaginably unfair for these boys. It's not long until I am 'officially' a stepmum but it only truly counts when you earn the recognition and trust of the children involved,' she added.
'My heart could burst.'
The mother of Freddy and his older brother Bobby, 14, is 38-year-old Jeff's former partner Jade Goody.
Jade tragically lost her life to cervical cancer on Mother's Day in 2009 when she was just 27.
After uploading a picture of her special card, Kate is then said to have shared a moving snap showing one of her husband-to-be's boys on the beach holding a 'mum' balloon.
She said: 'Happy Mother's Day to all of those not with us. Never forgotten. Fred and Bob's mummy brought the sun out for us.'
Jeff - who found fame while appearing on reality series Shipwrecked - asked for Kate's hand in marriage during a romantic holiday in Ibiza.
'Both boys were intrigued by how it would work, how I'd do it', Jeff explained to HELLO! magazine.
'They said, "What will you do, Dad, if she says no?". I said, "I don't want to be complacent, but I think we're alright".'
Bobby and Freddy will act as joint best men at the upcoming ceremony.