Why does that matter? She would learn with time. If her parent dies though, they are dead permanently. I'm not even 100% sure I know what you're arguing.
At that age children do not understand that parents are helpless in such situations. "But you could have rescued him!". They do not see the consequence of fathers death because for them its no option. To children parents are invincible, the answer "I could have died." is for young children not believable. For them it is the dog died and the parents didnt do anything.
And the dog is dead too, he's not coming back.
Children don't live in a world where "One day you will understand" holds any meaning, it's the here and now. You kick off an entire train of thought btw: Would daddy leave me in there too?
Of course none of this doesnt mean that rescuing the dog and giving your life is the right thing to do, but to claim it's an easy decision as you say isnt right either.
Besides animals such as dogs are
fantastic helpers to overcome grief.