I'm not thinking about you or your kids. I think it is unfair to this dog, who has already had a hard life, to introduce him to your home at this time. What if he needs special attention? What if he has health issues? What if he regresses or has separation anxiety? He deserves your full attention and not risking that he would need to go to yet another home. I know lots of people who have had to return rescue dogs for various reasons - I can't say I agree wiht doing so - but if there is the slightest changce it might happen, I say "No dog!"
By the way, have you had a dog ever? I grew up with dogs, and I am waiting until my kids are old enough to help - walk the dog safely, give the dog a bath, feed the dog as a chore, groom the dog several times a week and vacuum the shedded hair, etc. If you have never had to take care of a dog before, you need to educate yourselves about care and vet costs. A dog is not just a prop in family photos, he should be part of the family. I don't mean that as critically as it probably sounds, but wanting the experience of the dog is great, but it is also work, time, and expense.
Have the baby for several months, wait until you have a break from school - summer or at least a long holiday, and give the dog the attention he will deserve.
Yes, the dog will bring a lot of joy to your family, but your kids won't even remember getting him right now. I think it would be nice for them to have memories of bringing home this pet.
After the baby is born, consider adopting a cat - some of the same issues apply. But a cat can be kept in the home with no walks and just requires litter changes several times per week. A cat is loving and coddly and playful, but much more indepedent.
Good luck with your expanding family, be it 2 legs or 4!