I work with a family that has a child with down snydrome and when they travel without him they have to do the form. I recently went out of town and she told me about it. I never realized that you couldn't just type something up and give it to the person caring for your children. For it to be legal, it needs to be notarized and have all of your information on it (name, address, etc.). Make sure that you state the full names of the people responsible for your children as well as your full names. There is no need for SS#s as insurance no longer uses that as identifiers, but do put yours and your husbands DOB. There should be different forms online that you can find. Also copy front and back of your insurance cards and include them as well. Unfortuneately, this is what the times have come to that we have to do this, but better safe than sorry. You don't want your family memebers to be left out in the dark because they don't have the propper information.
I would include a list of drs numbers, any allergies to meds or food, etc. just for your family members but no need to include that on the letter.