We recently were able to switch about four ways to get four familes that had been scattered all over seated together. it must have been comical people switching all over the palce, and it went literally one seat at a time. It started with me. A lady a row back on the other side wanted me to switch my aisle for her middle so she could sit next to her eight year old. Nope, not gonna give up my aisle fot that one (had it been a three year old it would have been different.) But then I found out her DH was in an aisle in the front of the plane, so i offered to switch with him.
Once I got up to the front, I found that the guy in the aisle seat next to my DH and DS was flying solo. So I asked if he would switch so I could get into the same row. no problem. Then I found out taht the guy in the aisle seat in the same section as DH/DS was flying with the people in the seats in the first row I had switched to. So he switched with me, and then with the guy that had switched with me one switch previous.
In the meantime some similar switching was going on with a few people/families in the back of the plane. In the long run we were all with our respective families/flying companions.
Thta said, the FA's or gate agents WILL move people so children under about five don't have to sit alone. Keep in mind that they don't assign exit row seats untli you get to the airport, so that will free up some seats, often together. We always ask to move to that row now that DS is a strapping 17 year old.
Anne