Personally I'd cancel.
I found our puppies more work than our dcs, and I have absolutely vowed never to get another one (I will). Think incontinent nappieless toddlers on speed, with razor sharp teeth, and a tendency to nip anything that moves, and chew everything that doesn't. They reduced me to tears at times.
Add to that the joy of getting up numerous times in the night, to stand outside in the freezing cold while they sniff and play and resolutely refuse to wee. Only to come in, whereupon they immediate wee. Then poo. And don't forget separation anxiety. The whining, and crying and howling that can be heard 3 streets away, despite the fact you are only in the next room (cleaning up their latest poo.... having shut them somewhere else to stop them eating it).
Those are just the highlights.
That said, those aren't the reasons I'd cancel. The main reasons are that just as your newborn arrives your puppy will be entering into a really stubborn teenage phase, and your older child will be dealing with some pretty complex emotions as they adjust to your time and attention being largely focussed on someone other than them. It's a tricky period, and you will not have the time you need to invest in training your dog. Teenage dogs need a lot of refresher training sessions as they start to challenge, and ignore and generally be a complete pain in the butt. It's not a good time to suddenly ignore them if you want to have a balanced well adjusted family pet.
Definitely cancel and revisit the idea when your dc1 is old enough to do the majority of the dog care and training, and would enjoy the dog more.