I would trust it as much as I would trust any dog If its been trained everyday from 8 weeks old, to use its brain and get mental stimulation
I think the 'treat all dogs the same/don't trust any dog' approach is silly tbh.
I trust some breeds FAR more with my dc depending on the risk of injury/damage. Breed is one of the most important factors in a basic risk assessment.
I regularly leave my 5 year old alone in the same room with my MIL's dog when I pop upatairs or to the kitchen.
It's a Yorkie. Lovely, friendly nature. Unlikely to turn I think but if something 'went wrong' then I'm pretty certain that it's not capable of killing or severely injuring my 5 year old, who is more than capable of jumping up well out of its reach. It's a risk level I'm happy with.
Would I leave my 5 year old completely alone with a labrador? No. Because again, lovely though it might be and unlikely to turn, if something went wrong then they're big and strong and a 5 year old would be less likely to be able to get away.
I wouldn't even have my 5 year old in the same house as an XL Bully, even if it was on a lead at the other end of a room.