In football or basketball, you have a choice to use a "man on man" defense or "zone" defense. The first option is exactly what it sounds like. Each defender picks a player and sticks with them no matter where they go on the playing field. The second option requires a little more defensive coordination but correctly done, can be equally, if not better at covering players.
When Rachel went mobile and started walking, Yvonne and I could easily go with either defensive option because the parent-to-child ratio was 2:1. As a firstborn child, she was never out of our sight and wouldn't be further than an arms reach away.
The boys however were a different story. When they came along, the grown-ups were officially outnumbered. Now that the boys are walking, they find no pleasure in sticking together (no...that would be too easy for us). Instead, they almost always venture off in opposite directions.

Take a close look at this picture taken at the Stanford Shopping Center. It is said that a picture is worth a thousand words.... this is what I see when I look at the photo:
- You can't really see it in the photo but Nathan (on the right) is peering through the bars and contemplating how he can get into the water on the other side.
- Seeing that I'm focused on taking a picture of Nathan, Daniel takes off in a sprint (Yvonne was on the other side of the planter box getting Rachel situated in a stroller)
- The relaxed (probably child-less) couple approaching Daniel are likely on a nice stroll through the mall without a care in the world. They're probably thinking, "Aww, look at this kid stumbling towards us....Where are his parents? Is he alone? How irresponsible of the parents to just let the kid wander off....can't they control him? Maybe we should call the police."
- At this point I shoot Yvonne a one-second look that basically says, "We got a runner, you have Nathan and Rachel, I'm going after Daniel." Or, in defensive talk, "Shift your zone coverage over here because I'm breaking cover to go after the runner..."
- I finally catch up to Daniel, scoop him up, and give a polite smile to the couple. They look at me and are probably thinking, "When we have kids, we'll never be irresponsible and let them take off like that..."
Here's the kicker: This is just about a daily occurrence for us.