I'm not a parent, but I suspect that if you're driving your kid(s) to the ER, there's a risk of having hours or minutes left with your kid(s) before they perish. So while it is riskier to call on the way, the risk of your spouse missing out on the last few moments of your child's time on Earth is actually greater.
That being said, one would hope that is a seriously unusual edge case for requiring voice commands.
(I personally find it awkward to use them, and still don't trust them to get it right, so I try not to use it ever unless I have no other choice.)
I think I'd always call an ambulance if it is what I perceive as a life threatening situation. Actually I did it once. You can't give CPR to your kids while driving. I also think calling the other parent would be way below in term of priority than making sure my kid stays alive so I think I would call only when ER specialists or at least paramedics have taken charge of them and that may be some hospital clerk doing it for me if I have to be available to give information about medical history.
That being said, one would hope that is a seriously unusual edge case for requiring voice commands.
(I personally find it awkward to use them, and still don't trust them to get it right, so I try not to use it ever unless I have no other choice.)