traveling to U.S. with children from another family?

Do I have to get a letter signed by a lawyer to take children from another family into the U.S.?

What you need is a letter from the parents giving you permission to take them out of their home country and into the U.S. It should be signed by both parents and notarized. You may or may not be asked for it, but should have it. If you are Canadian, see site below.