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.
In terms of how an estate plan can provide for “after-born” children, there are a few drafting techniques that can address this issue. For example, in your will, it would refer to your current children typically by name and their date of birth. Then, your will would provide that any reference to the term "your children" would include any children born to you, or adopted by you, after the date you sign your will.
In addition, in the section or article of your will that provides how your estate and assets will be divided, it could simply provide that your estate and assets will be divided into separate and equal shares, one each for "your children." That would mean that whatever children you have at the time of your death would receive a share and thus the will would work as you intend, even if you did not amend it after having a new child.
On a side note, you should make certain that your plan does not give the children their share of your estate outright while they are still young. Rather, your will or living trust should provide that the assets and money are held in a trust structure until they are reach a certain age or achieve certain milestones such as college graduation or marriage. Any good estate planning attorney should be able to advise you about this and help walk you through the various options you have available to you.