Nikki Bella says she and her new fiance, John Cena, have already decided on their family plan -- NO KIDS ... because the WWE superstar has no desire to be a dad. 

Cena's talked about not wanting children in the past ... but he also swore he'd never get engaged -- so, now that he's changed his mind on marriage, we were wondering if kids were now on the table as well. 

There's more ... Nikki also tells us what she has in mind for her bachelorette party -- and why she was SHOCKED her man popped the question at WrestleMania.