How to tell the step-child?

My fiancé and I haven't even formally announced our engagement yet (I'm wearing the ring and our closest know, but the kids, his ex, FB world, etc stays on the DL a while longer). This is bc he is still working through some sensitive issues with his ex (whom I respect and she & I hve become friends). 
How do we tell the kids? I am tired all the time, 9 weeks and nearly showing already. It's got to happen in the next month. Do we tell the ex first, then sit both kids down together and tell them about both the engagement and the baby? 
Everyone's amicable, and it's no surprise where the relationship is going. The kids already love each other like siblings, and my 6 yr old calls his 10 yr old "sissy". Just want to make sure we handle this delicate time appropriately.