You want the document to be specific enough that they won't get turned down at any point during their travels or denied medical treatment, but you also want to keep it vague enough that should the travel plans change (due to delays, cancellations, a sickness or other unforeseen emergency), they won't encounter problems either.
This document is very easy to obtain. You can get it from anywhere there is a notary. I know a lot of banks will provide notarial services for free or a very small fee for their clients who have an account with them. The post office also offers this service, as do lawyers offices, and the fee is usually no more than $5-$10.
A very simple but brief letter is all that is needed. It should include the name of the parents, the name of who is allowed to take the child, the childs name name, and what is allowed. An example (you can modify it to fit your situation):
We, _________ and _____________, give _____________ and ______________ our full consent to travel alone with our child __________________. We also give ______________ and __________________ our full and unconditional consent to seek and obtain any and all medical attention for our child ________________ that they deem necessary.
All you must do is have the letter typed up (but not signed). The parents must take the letter, their drivers licenses (or other form of picture identification), and then they will have to sign and date it in front of the notary. It's a very simple, fast, and easy process.
If there is no additional fee (and there usually isn't) get at least 1-3 extra copies. Why? Well, the people traveling with your child will need to keep the original with them at all times for the length of the travel. I always like to keep the original plus 1-2 extra with me in case something happens to one, or an airline employee takes one. You should also keep a copy with you so that if need be you can fax it or send it to anyone if the need it at any point.
Those traveling with your child should also bring a notarized copy of your childs birth certificate. This will help to verify that you (the parents) are truly the people who signed the consent form. Airlines also use this to verify a childs age.
Those traveling with your child will also need to have your childs passport and possibly a visa for them if it is an international flight. Another thing you should have them take is your childs immunization book. That way if any emergency should arise (no matter where they are) the treating physician will at least have some verifiable information to go on. This book also usually includes what your child is allergic to, and some small but very helpful notes from their doctor. Be sure those traveling with your child keep the name and number of your childs doctor handy. Along with this, be sure they bring your childs Medical ID card.