Reading a bunch of the news stories about the case, I kinda get the feeling that sending the case to a grand jury was just standard procedure. I can't find anyone--not even those who would ostensibly be on the "prosecution" side of this case (law enforcement, the district attorney, ...) who believed the father deserved to be charged or prosecuted. The local news stories and statements by all concerned treat it's going to the grand jury as unremarkable, and just the next (last) step in the process to clear the father of any legal wrongdoing...
While I'm with you on it being a waste of money--and certainly burdensome on the father if he was obligated to pay for a lawyer for this (not to mention other "potential defendants" against whom no one in the law enforcement / legal community believes charges should be filed)--I'm not sure this is a moral or legal injustice, or that the district attorney is personally or professionally at fault, at least based on the way most of the news stories and statements by those involved and those familiar read...
(I submitted comments to a few local news stories asking for clarification on this... I'll comment here again, if I see anything that speaks to the issue one way or the other...)
Got a reply to my query from a reporter in TX: "in Texas, every case that is ruled a "homicide" -- death by other than natural causes -- is routinely forwarded to a grand jury even when prosecution is unlikely." - Grand Jury: father's action was justified | The Gonzales Cannon