There are many things an executor cannot do. Legally, the executor has a fiduciary duty to execute all elements of the will in accordance with the law and to the best of their ability. After someone is deceased, the executor cannot legally change the names of the beneficiaries in the will. So if you or your grandchildren were named as beneficiaries, there is nothing the executor can do to change the will.
Similarly, the executor cannot stop any beneficiaries from contesting the will. If a beneficiary does not believe that the will was accurate or they wish to challenge elements of the will, they can file a dispute through probate court, and the executor cannot stop them from doing so.
The executor also cannot sign the will on behalf of the deceased. If your family member passed away without signing their will, even if the unsigned will stipulates who the executor should be, it will not legally hold up in court because they did not sign it.
Finally, the executor cannot begin distributing assets until such time as the person who wrote the will has passed away.