@Ollifer "I've been suicidal before but I'd never do it because of my child"
I'm incredibly sorry that you reached those depths of depression, and incredibly impressed that you managed to pull yourself out of it, but unfortunately not everyone does.
In the OPs scenario, you have parent A & parent B, each taking 50% care of their joint DC.
Parent B has openly declared they're suicidal.
If you were parent A would you risk the potential deaths or life-long trauma of your DC knowing this?
Would you say to parent A: "Oh, you feel like killing yourself? Never mind." Whilst strapping your kids into the back of their car?
Of course you wouldn't.
No good parent puts their children at risk of trauma, harm and certainly not death.
Parent A should ensure that parent B gets all the help and support they need to help them recover, whilst taking full care of the DC to ensure that they are not exposed to their parent trying (& possibly succeeding) to kill themselves.
For you, @Ollifer , feeling suicidal didn't actually mean committing suicide, but I personally wouldn't risk my kids being in your (or anyone else's) care if you'd told me that you were; surely you must understand that?