Automatic grid layout isn't supported, but you can use top and left and expand=0 in the parent style to place the child objects and stop parent resizing:
# ignore: Parent,top,left,width,height
ID,Name,Parent,top,left,width,height,shape
Parent,Parent,,0,0,40,40,rect
Chlld1,Chlld1,Parent,0,0,100,100,rect
Chlld2,Chlld2,Parent,100,0,100,100,rect
Chlld3,Chlld3,Parent,0,100,100,100,rect
Chlld4,Chlld4,Parent,100,100,100,100,rect
We'll fix that offset of child objects and add support for width/height auto and padding for groups in the next release.