I find the command "wait" would be perfect for a situation like this.
Here's how you teach it:
1. Put your dog on a leash and head towards the door like you're going outside. If you have a screen door, prop this open ahead of time.
2. As you get to the door, tighten the leash and say "wait," then opening the door. When/if he tries to dart outside, give a sharp "no" or "ah-ah" and use alternating pressure (tightening the leash to pull him back and then loosening it again) to keep him inside. You have to be quick. Don't let his paws cross the threshold of the door.
3. Keep practising this a few times until he finally "WAITS." Once he gets it, praise him and then go back into the house for a break (or go outside for a walk after saying something like, "okay, let's go.")
4. After you practise this for a few days and you think he has it, make one change. As you approach the door, do NOT tighten up the leash. Leave it completely loose. Tell him "wait" as you open the door. If he tries to dart out, do like you did before and snap him back into the house with a firm "ah-ah" or "no."
5. Once he has this down pat for a few days, it's time to add distractions. Have one of your kids walk by you (without looking at or acknowledging the dog) to go outside and have them play ball or do things that the usually do outside. Pretend to talk to someone at the door, sit in a chair and read a book by the door, move around and hum or sing. The dog is learning that if you tell him "wait," no matter what, if you don't tell him "okay, let's go," he is NOT to step over the threshold of the door.
6. Graduate to a longer leash (say a 20ft. one). Open the door, tell the dog to "wait" and then you can walk around the house, the dog can follow you, or just may want to sit by the door and peek outside. Still, like previous times, he is NOT to cross the threshold. Once he gets this, do like last time with the short leash and repeat all the distractions.
7. Once this level is achieved, switch back to the short leash. YOU are going to go outside. As you get to the door, say "wait" and open it. YOU will step over the threshold, holding the leash up and back to keep your dog on his side of the door. As soon as you are outside, turn and face him. He should still be on his side of the door. Tell him "wait" and loosen the leash. If he tries to rush out to join you, the second his toe crosses the threshold you use the leash to snap him back inside the house, telling him to "wait" again. But you stay put on the other side! Once he gets it, softly praise him. Hold up your hand can sometimes be a good reminder to "wait." Finally, use the release words ("okay, let's go") to allow him to come over the threshold to join you outside.
8. Practise by getting further and further away from the door and by adding more tempting distractions. Over time, he'll understand not to cross the threshold if someone tells him "wait."
I hope this is some information you find helpful and I hope it wasn't too long or tedious.
You can also apply "wait" to other things, too, like the car or maybe even the boundary of your front lawn and the road.
Good luck and keep us posted on when the new baby arrives!