Hi and welcome to YT! Loving your user name, "AwwJackson", bc when I looked at his cute pics, I went Awwwwww! You nailed it.
Hmmm...I wonder why he'd done fairly well w/ past kids but got anxious w/ these...what was different...the environment or the kids (ie, higher energy than normal, loud, bold, anything...? Boys instead of girls?)...some of those things may be clues to what's going on.
I'm no trainer and I think this is more of an issue for a trainer or someone good in training. Wish I knew more. I'd probably continue safely exposing him as you are now as that seems helpful. Maybe get involved in an obedience class too?