A power of attorney will not work, nor will guardianship as the nother will have to agree to that as well. The only thing that you can try is a notarized letter from the father indicating that you have authority to make decisions in his absence. Then it would be up to whatever institution or individual to decide if they will allow you to make those decisions.
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A power of attorney will not get you what you want. A power of attorney will only give you authority to act on behalf of someone when it comes to assets and finances. What you want is a guardianship. You can have your husband simply sign a guardianship affidavit or give you authorization to act on their behalf with respect to school and medical issues. This is very easy to do and incredibly inexpensive. In fact, for physicians, your husband need sign an authorization that can be provided by the physicians office indicating that you are authorized to act on behalf of the children.