Just the lens will cost the entire $700.  I like the pentax bodies for their quality and weather resistance.  
For sports your looking for a zoom.  Something like a 60-250mm would be ideal.  60mm allows you to get the entire field, and 250mm zooms in on the kids.  135mm isn't really much zoom and I'd prefer more like 250mm-300mm.  When you get into 400-500mm it's a heavy lens requiring a tripod and not great for hand photos.  
For the night aspect you want a fast lens with a large aperture.  Cheaper lenses will be something like F3.5-6.5 for example.  So when zoomed in at 250mm it's equivalent to F6.5.  A more expensive zoom will have a fixed aperture like F4 across the entire range.  F4 vs 6.5 makes a big difference at night and lets in way more light.  
Pentax has a 60-250mm F4 that is weather resistant for about $1200.     Canon has a 70-200mm F4 for $600.  There is plenty available.  A $1200 lens will have better image stabilization and autofocus.  
Tamron makes a 70-200mm F2.8 in Nikon, Canon, Sony, and Pentax mounts for $750.  The F2.8 is awesome and would give you an intro into night photography without breaking the bank.  But buyer beware, autofocus and tracking can be a problem with lower prices optics.  
Check out bhphotovideo.com to see what's available and pricing.