Your enthusiasm is contagious! Congratulations for you and your family. I'll share with you a technique I used to announce my third (surprise) pregnancy with my husband and two sons (aged 2.5 and 4 at the time).
I got a white t-shirt that fit my 2.5 year old son, Paul, and painted on it: "Now I'M the big brother!"
I had Paul wear it, and he thought it was great, but he didn't get what it meant. He found it handy to taunt his big brother, but that's about it. Then when Dad got home from work, I asked him to take a look at Paul's t-shirt. He did, but he didn't get it at all. So then I put the pregnancy test on the counter. BINGO! Lightbulb clicked on. That moment was priceless. And yes, we told the children a few weeks later, once we had been to our first doctor's visit.
Best of luck to you!