First off, Congrats!!!!!!
Second, Don't change anything. I'm not joking, I'm dead serious, the kids will get used to it. My son turned 17 last month, and my daughter will be 16 in about two weeks. I never changed anything, and cranking up a 100 watt head never bothered them. I used to have band practice in my basement, where we had Drums, two halfstacks, an ampeg SVT 4 and two 410's and full vocals running through a mixer. Underneath their bedrooms. They never had a problem sleeping, and will tell you now it was fun times having band practice there.
Sure, you have to start off low, especially when they're first born, but I think you'll be amazed if you play as normal for the next few months while she's pregnant, and build up, how it won't affect the baby at all.
Oh yeah, Congrats again.

best thing that ever happened to me was becoming a dad. They're my life.
Does it affect you and your wife now? it won't affect the kids. They'll grow up with it, and be used to it, just like you and your wife are.
for the first 2 1/2 years, my wife worked until midnight and it was just me and the kids. Id have a head and 4x12 in one room playing at sane volumes, and them sound asleep in the next room.