Just to give you a perspective from the opposite side of this situation. We have friends who had their first child recently. We don't see them as often as we did before the kid was born. We don't have children, so under your theory, we shouldn't be that busy. But, we are...just not with kids. Maybe your friends haven't been around to hang out because they know you are busy and don't want to bother you. Or, no offense, but is it possible that they felt like they needed to distance themselves from you for a while. There could be any number of reasons, some related to you and some unrelated. New parents can, at times, be difficult to deal with when you don't have kids of your own and you don't understand what they are going through. Or, it could just be as simple as your tastes or interests have changed now that you have a baby and they just don't enjoy spending as much time with you as before. That isn't a reflection of how they feel about you.
I'd take the high road in this situation - invite them over for dinner and explain that you're starting to make plans for the baptism and you want to see if they are still interested in being God Parents. If they are, be very clear that you want God Parents to have an active role in your child's life (be specific about what you mean). Then, give them the opportunity to change their minds. They don't have to make the decision right then and there. Tell them to think it over and let you know in a week or two. Be very clear that whatever they decide is okay with you.
Another option - if you decide for sure that you don't want them as God Parents, then invite them to dinner and just be honest and explain that after giving the situation more thought, you've decided to ask someone else to be the GP. Explain that this is in no way a reflection of your friendship.
No matter what you decide to do, if I was the one you asked to be a GP and then changed your mind, I'd be more hurt if you didn't talk to me about it. As your friend, I would deserve that courtesy, at least.