@Frank *********
this would be a Non-immigrant O follower visa. This person would follow some one on a Non-immigrant O Visa volunteering at a foundation.
The affidavit just states I am married to "spouses name" and the following are my children. (With additional basic information). In the past you take this affidavit to the US embassy/consulate and have it notarized. I June the US embassy and consulate stopped offering notaries for affidavits like this.