The auto-focus (AF) focused on the trees in the background. In future, there are a couple of solutions around this problem .. if the camera has AF lock, you can focus on one of the kids then shift the camera to the center. AF is locked would keep the kids in focus. If you don't have AF lock, you can focus on a kid then turn AF off. The other thing depends on whether you can manually set a AF point. In my camera, I can set it to the left/right of center; perhaps you can too ... now the camera locks on a kid without moving the camera.
A wide aperture, like f2, is going to create a shallow depth of field. You'll have to experiment with your lens to determine the depth of field at various aperture settings. One idea is to photograph with a tape measure where you can see individual mm's or cm's marked out. Pull the tape measure out 3-4m and photograph along it at various distances and aperture settings. You can tell by the blurriness of the lines where the depth of field will be by how sharp the markings are on the tape measure.
You'll want to test the the focusing distance at different aperture settings because different lenses, especially those that have a minimum focus distance depending on zoom, will frequently also have different minimum focusing distances too.
The only way to know is to practice/experiment with the lens and get comfortable with how it operates.