This question presents a fairly common issue posed to estate planning attorneys. The solution is also pretty easy to address in your will, trust and other estate planning documents, including any guardianship appointment for your minor children.
First, its important to note that you should not delay establishing an estate plan pending the birth of a new child. In fact, if your planning is done right you most likely will not need to modify your estate plan after a new child is born. The problem with waiting is that you cannot know what tomorrow will bring and you could die, or become incapacitated and not having any type of plan is a bad idea.