outrageous. How insulting to your partner
Unless there is something you're not telling us, the bottom line seems to be that you think they are your children, you somehow have the right to remove his privilege of looking after his own children, and only your risk assessment is valid
It's not actually that outragious, insulting it may be but outragious it's not.
On our area HUB referal forms we have a tick box section with a few fairly standard reason for referal, under 10yo's being left home alone is one of those things.
It also has a section which asks about discussion with parents with a Y/N and outcome of discussion bit on it, that has a few tick boxes along with space to write stuff if a tick box is not relevant, parent lacks understanding of concerns is one of the choices.
If both are ticked a visit would occur, one of the things that would be asked of the other parent during the visit would be "what are you doing to prevent this in future" obviously that would be if the parent who didn't agree wouldn't stop it.
If both parents didnt agree with each other then the onus would be on the others ability and willingness to mitigate the risk of re occurance.
A one off whilst prompting a visit would be unlikely to involve any further involvement but future risk could.
You would be surprised at the amount of situations where if one parent is a bit lapse but if the other is not then that one will be considered to have mitigated any problem, same as when you have one whose very crap but if another family member goes behind them doing the stuff they don't bother with then the kids needs will be being met.