Speak to a family law attorney about the particular facts of your case. Grandparents have standing to fight for custody of a grandchild under certain circumstances. A power of attorney alone does not protect parent rights. A power of attorney is giving another person the right to act on your behalf. A guadianship would be something that gives another person rights over your child and/or your assets.
A power of attorney probably has no bearing on grandparents' rights (which are very restricted in Texas). I would urge you to consult with a good family law attorney in your area. I am very familiar with the courts in Dallas County and can assure you that they will carefully follow the law on this matter, so you'll want to make sure you have a good case ready to go before you appear there.
Sign up to receive a 3-part series of useful information and advice about child custody law.