Hi op. Sounds like you’re struggling right now and need some moral support from your family and friends. Make the calls when you need to, get some space when you need. Are you okay?
If you leave them play, just keep checking on them. Make sure there is nothing dangerous around. They are at an age where they will wander off and you can’t always watch them, so try not to stress about this.
Biggest problem is safety around stairs, small toys etc. But you know this and it’s not as if you left the house. You were right next door. You could hear them.
Don’t panic! Next time, just keep popping back in. Make sure they have a safe space. No choking or falling hazards. Same applies when you have to get something from another room, go the bathroom etc.
I would also ask for support from family to come and give you a break. If you’re having anxiety, you can always talk to your gp.
Mum guilt is there for a reason. It reminds us to be aware of dangers, so you will probably be more aware of it now and won’t leave them unchecked for any length of time.
I’m more concerned about you. I hope everything is okay 