Another option would be asking someone on the plane to switch seats so you could all sit together. It depends on where your seats are and how small your child is. If you have a seat in the middle, you're probably not going to find someone who will switch. If you've got a window or an aisle, you might. If you have a small child, under age 5, you'll probably be able to ask whoever's next to your child to switch.
We fly standby and rarely get seats all together (2 adults and 2 kids). However, with young kids, the flight attendants help us find at least 2 seats together, and usually someone will move so one of us can sit with the older one, too. I've found that when given the option of sitting next to a small child on a long flight or moving to another seat, people usually chose to move. So a big thank you from me to the people who do that so my 3 year old isn't sitting by herself.
But not sitting together isn't the worst option, either. When we just had one child, one of us would sit with him, and the other would sit by themselves in the seat we were given. We've found it's actually nice to have a seat by ourselves for a bit, I can actually watch the movie or read for a while, and we'll switch every 2 hours or so.
We were split up 2 and 2 flying back from Germany this summer. When the flight attendant realized this halfway through the flight, he said we should have spoken up, he could have gotten us seats together. I told him we were coming off of a 2 week vacation together, being split up wasn't so bad for a while.