I didn't read all the post here so I'm sorry if this was covered, but... You're most likely setup in 'matrix' mode for metering. Matrix mode looks at all the dark and bright areas in a picture and tries to find a happy medium. That's why the image is under-exposed on the kids and tank.
If you change the metering to spot, the camera will adjust the exposure for whatever you have inside the hash marks in the center of the screen. Center weighted evaluates a slightly larger area than spot, but is only for the center (evaluates mostly inside the center box but gives concideration to larger circle surrounding... think of it as having the center mark being 2-3X bigger)
Here's how to shoot (if you're not going to use the HDR Mode, which for a shot like this, you really should!!!). If you are only concerned with the kids faces use spot... for kids faces and whats directly around then (tank) use center-weighted..... NOW place the hash marks (center of screen) on one of the kids faces and hold the shutter button down and dont let go, you will hear a beep and the hash marks will turn green.... You now have Exposure and Focus locked, you can now move the camera to compose your shot (meaning get the clouds, tank and kids to where they look best on screen... not necessarily in the center), then release the shutter button and the picture will be taken.
Other modes to use would be face detection which I believe (don't quote me on this) automatically exposes for faces when detected. Also get familiar with Sports (action) Mode, It sets the shutter speed higher so you don't get blurry shots, the down sode is the shots may have a slight bit more noise cause it may have to bump up the ISO if lighting is not ideal. another consideration for shots where the background is brighter than the subject is to use the Back-light Mode... This uses the flash to fill in your subjects and exposes for the background (this is usually not good for distances of more than 10 feet (depending on the power of the flash) and can introduce weird coloring if the ambient light is not similar to the flash).
But the best thing for you to do right now is just use HDR.