if you’re not already doing so, shoot raw & edit in lightroom instead of letting the camera process the photo for you. You have FAR more control over your work.
There is a time and place for off-angle horizon line shots, but I’m not sure portraits of children is where is works best. Instead, watch a few videos on cropping using Golden Ratio or Rule of Thirds or Fibonacci Spiral to really draw the viewer in.
I LOVE your FB cover shot! 🙂