How to improve your photo
Be mindful of your cropping points, try to avoid cropping at the joints. Crop at midpoints to let our mind complete the shapes. Also consider moving closer to your subject, fill the frame with the laughter and eye contact. Wait for eye contact from both your subjects, the older child keeps me on the page with his glance, the younger child makes me want to see what he is looking at.
Avoid using direct flash for portraits, frontal lighting is not very flattering and it flattens the image. Add depth by using side light, and especially window light. Try to make this portrait again, place your subjects near a window, watch the way the light wraps around their face creating a 3-D look to our 2-D images. Be sure to exclude the window from your frame.
When making portraits, lenses in the 50mm-80mm range help us avoid distortion, provide separation for the subject from the background and allow us to work close enough to make the connections you are obviously good at making.