He was supposed to have 7 hours of supervised visitation in Fairfield County that Saturday, so it's possible that he knew they were planning on coming back from NY. It was a holiday weekend so perhaps JD communicated something about their plans for the time of getting the kids to him on Saturday. Although, JD seems to have been very cautious about getting information to him, and there was a court appointed supervisor for the visit so its likely they communicated plans through that person.Did he know that his children were going to be taken to their grandmother's in NYC that day? Did he think she'd just hand them over to him after their mother and her daughter went missing? (I'm aware that he tried to see them a couple of days later.)
More likely, FD probably didn't know that they planned to go to NY, because JD missing her appointments set off the search for her far earlier, and far more concerning, than he might have hoped, and the kids were already with JDs mom. If he thought they were just chilling at home in NC that day, she might not have been noticed as missing until after school or even later, and then by his Saturday morning planned visitation the gears wouldn't have started churning yet and the kids would be available for him to pick up. But they were already with JDs mom in NY.
Some have surmised that FD kept his phone on during the Hartford trash run because he was expecting a call to come get the kids since JD wasn't around.
Remember that he initially pressed to keep the custody hearing date that week even after she was reported missing, so he was clearly banking on buying some time with a runaway/mental health breakdown/abduction while running at Waveny hoax for JDs disappearance.
I think it all went south for him far faster than he expected.