It wasn't entirely physical harm Nathan worried for, it was the influence he knew Angela had as their grandmother and what he knew, personally, that those emotions in a kid (or adult) could do. He wouldn't subject his sons to that kind of conflict and he wouldn't let them come to any harm. He'd begun to grow used to these visits, however, playing nicely for the sake of Simon and Monty, hanging out on the peripheral of the scenes to give them time together. It was, perhaps, quietly serving to settle the sharpest edges of anger and resentment in him, though it certainly hadn't resolved anything.
The question wasn't a complete surprise, as he'd been expecting for the last few visits she might broach the subject again. "Yeah, gimme a minute," he said, then turned to address the boys. "All right, go find Mom, tell her I'll be there after I talk to Grandma, then go get ready for your lessons with Sarah, okay?"
There was some slightly hemming and hawing about school in the summertime, as usual, but he sent them on their way down the hall with a smile, watching as they turned the corner to go down the one their rooms were all in. He waited, listening for the sounds of a door opening and the boys talking to Heidi before turned back into the room to face Angela.