Given that OP didn't know whst happened until now, no her kids didn't see it.
If he was arrested near the house, it's possible that the incident woke them and they saw out their bedroom windows. It's unlikely, but it's possible. Having been in that situation, it was my first thought.
This is so OTT. The children do not need to know any of this.
The 9-year-old is already aware that their father was missing. Would you suggest lying if the child asks?
I can see why some would think my ideas are over the top, but having been a child in the situation, I don't think there are benefits to keeping children in the dark.
One of the worst things when dealing with a parent arrested for shite like this is knowing something's gone wrong but the adults wouldn't talk about it to my siblings and I and being expected to just act like everything is fine. That was considered best then, it isn't now. The children's questions, feelings, concerns, and general thoughts should be heard and addressed and there are age appropriate ways to discuss this.
It isn't the worst, most unforgivable thing ever, but for me it would cross a line. I do not take lightly when people's drug use choices impacts the rest of the household, and this has. I might be 'up my own arse', but I'd expect at bare minimum for him to answer the questions our children had on the topic and be able to discuss it with them appropriately as part of taking responsibility which to me is a key part of any apology being real.