Get the Nikon D3100, you should only consider the J1 if its really important for the camera to be small. The Nikon D3100 will get much better picture quality especially in low light, and will have many more lenses available.
To get good photos of your son on the soccer field you should get a zoom lens. Here are some zoom lenses for the D3100 from our sister site LensHero you could consider!


