The best way to do this is to contact an estate planning lawyer. Most lawyers can take care of a Will in less than a day or two.
Good luck and have a great trip!
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Mr Frederick offers sound advice, but there is more to this than drafting a simple will. To have an estate plan done encompasses much more. When people do this often times they never go back to do full estate planning, so be careful here.
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From the way you phrased your question, it sounds like you would like to add your nieces and nephewss as contingent beneficiaries to a Will you already have, in which you named your beneficiaries and Executor (who happen to be traveling to China with you.) You would want to consider amending your Will by signing a Codicil in which you would add your nieces and nephews as contingent beneficiaries should some unforseen tragedy occur on your trip that would preclude your beneficiaries from receiving their bequest under your current Will. You would also want to add a contingent Executor in case your currently named Executor was unable to act. Usually, it is wise to name a contingent Excecutor (in Maryland called a Personal Representative) and contingent beneficiares as a matter of course when establishing your Last Will and Testament. I agree with the attorney who previously wrote that many lawyers would be most willing to prepare a Codicil of the nature you describe with a very quick turn around time. I also agree with the other attorney who wrote that you may want to have a complete plan prepared, including a power of attorney, medical directive and so forth. Funny how traveling abroad is often the impetus for people to get their plans in shape! Have a great time on your trip!