In addition to what was said there is too much of a shallow depth of field.
On the subject of flash use, avoid using flash as primary light source. Flash works well as fill light in bright back-light situations. Utilize artificial lights, exposure compensation, flash compensation, ISO, and other camera settings to achieve the desired lighting and exposure.
Adobe told you why it was rejected. We may be able to help you get more detail if you will tell us exactly what they said. You send a model release of course?
Thanks for giving us the chance to consider your image. Unfortunately, during our review we found that it contains one or more technical issues, so we can't accept it into our collection.
There is colour and luminary noise in the darker areas. From the composition point of view it would have been better to include some space above the child's head. The DOF disturbs me:
It looks like there was some manual editing that turned bad.
In portraits, it's always important to get the eyes right. You should work the eyes, first to get the pupils black, and the brown bright. You should also correct the flash reflection. There is chromatic aberration. I would stamp it out and add a fake reflection in Photoshop... Don't use that direct in-camera flash for portraits. The reflections are not nice.