Don't worry about it! It's a frustrating situation, to be sure - and like I said the dogs I've taken in that were potentially dangerous weren't ever around children - I never would have taken them if I'd had kids.
The trainer is probably correct - most aggression is from fear (either fear of being harmed, or fear that the dog's status in the household is threatened.)
Best of luck. And FWIW, I had a foster Chow mix who was fine (I thought, my bad) for two weeks, then viciously attacked two of my other dogs causing a great deal of damage. I hated to do it, but she was out of the house the very next morning back to rescue, who probably had her put to sleep because they would have been liable for any future attacks if they'd placed her again. Some dogs are just unstable, either because of upbringing or faulty "wiring."
Best of luck with this guy, he's a challenge for sure. One learns more from challenging situations though.
Do post how it goes.