Gosh, OP, I feel sorry for all the bashing you're getting on here.
We are all going to make mistakes as parents - I don't think you need telling what a monumental mistake this was, you and your wife will both recognise that already.
You don't need a parenting course, or counselling.
I think you need to unpick why your wife left the kids alone in the bath, not from the obvious danger point of view, she has learnt a massive lesson on that from the trauma you have been through, and if her head was in the right place, and she was properly "present" in that moment, then she would not have left them - so I'm talking more from her own wellbeing point of view.
Was she tired? Had she had enough that day? Is she feeling ok in herself?
Please don't beat yourself up, your immediate reaction, and gut instinct kicked in, and you saved your daughters life. Not only that, you then proceeded to take every necessary measure to ensure that she was safe by calling 111, and taking her to A&E.
Yes, your wife made a massive mistake, but she did not endanger your children intentionally, sure she should never have left them, but she did not walk out of the bathroom intending for the events that followed, so please keep that in perspective.
You have both been through a traumatic event, and will still be in shock, try to be kind to one another, you need to forgive her, and she needs to forgive herself.
You've said you've always felt nothing but trust in her, her self confidence will be shattered, don't let it shatter all that trust as well, you can't let this one monumental mistake erode all the positive qualities she has as a Mother.
Focus on one another, and your children, rebuild whatever has been broken.
Sit and play some games with the kids, have a little teddy bears picnic in the living room, and move on.
Onwards and upwards OP!