He should be charged with negligence
3-year-old Kalli got her hands on a .40-caliber pistol and shot herself.
If you cannot keep a firearm out of reach of a 3 year old, you shouldn't own one, cop or not.
I'm sure he is devastated with the loss of his child, but he is the adult, and he should have kept better control of his weapon.
If you have small children, there are any number of products that secure the weapon, yet make it readily available if needed.
When I had small children living with me, I had 2 house guns, an old Hi Standard 12 gauge pump with a magazine lock out (When the lever was flipped you could cycle the gun, and pull the trigger all you wanted, and it wouldn't load) I kept a Browning HP unloaded, but cocked and locked, in a drawer by the bed, with the loaded magazine in a separate drawer. Even if they found both, they not only had too insert the mag, but also push the safety off, and rack it to get a round in it.
I also educated them, I took them to the range, and shot milk jugs full of water, with the 12 gauge, exploded warm cans of pop, and cantaloupe with the 9mm. I still remember the wide eyed look on their faces when I held the melon up, and explained thats what could happen to them if they played with guns. Sometimes a bit of fear, is the first step in respect. But at 4 and 6 they wouldn't touch a firearm without asking first.