I'm not saying anything different from all of the above. My cycling is not as much as it used to be but I can say it is much better having kids! Each kid gets his inaugural ride (well, #3 did not) when they are about a month old, in a sling on my road bike. Just up and down the street, but my wife gets a picture of it. My road bike has a trailer permanently attached to it. I am a stronger rider now for it, and pretty much don't recognize how quick and maneuverable my road bike is on the few occasions I take the trailer off.
I honestly don't understand why you are getting rid of the tandem. It has taken me 10 years to get my wife excited about tandeming. She has finally seen how much fun the kids and I have on the few tours we do each year. She finally got on a decent tandem and experienced the nuiance of the different models. Now, we are working to save up for our first tandem. I wish we had it sooner. We really could use a triplet or a quad at this point, but we are thinking of opting for maybe 2 tandems to feed the brood.
Keep the tandem if you both love cycling. The trailers are the best. If your kids are like mine, they will be screaming to get in and go for a ride....literally. As toddlers, they will try to climb in the trailer when you are going to the car to take them somewhere. Our oldest just got off his training wheels and is now selecting where we will ride. I am looking forward to having the choice of stokers between my oldest and my wife. Fortunately for my wife and me, we are close enough in height that either of us will be able to captain or stoke. I am not a fan of tagalongs, but I may have to do that as the others grow.
At least keep the bike for one season after the birth. Look into the local club family rides. You will not regret it.
Congrats on the baby. Here's to its health.