I would say that as long as the room is completely childproof and you have child gates preventing any access up from the playroom then you should be fine to just leave them there. You will be gone for all of a minute or two and if it makes you feel reassured at first, you could place them in highchairs to be sure they don't get up to no good but I don't really think it would be necessary unless you have a child who you are already having behavioural issues with who is likely to cause trouble when you are out of sight.

Most parents wouldn't have a room as child safe as a daycare setting and would be making dinner answering the phone, doing laundry and what happens when they need to visit the washroom? How do you manage to go to the bathroom? I think if you set your expectations of behaviour right from the start, they will quickly learn what is acceptable behaviour and if you are gone for a moment or two they will not bat an eye lid knowing what is expected of them. If they have the skills already to play independently versus being constantly entertained, then the chances are they would be far to busy having fun to even notice you were gone for that short time.