if placed on improperly (not depressed hard enough to stick to the trigger guard hard enough) it is definately possible to pull the trigger on a firearm.
But if placed on securely and properly it should be impossible.
I say the only stupid question is the one not asked. And I think anything is possible that's why I rope the action and trigger lock the trigger. For the cost of both and the time to put them on is minuscule compared to the alternatives of taking the chance. You can never be to safe and Murphy always manages to work his way into all equations.
the problem is they are not trigger guard specific. only gun i own that a trigger lock wont let me fire is the kids savage rascal.
but like posted, they are only child proof. last time i was at the range a guy forgot his keys, i lent him a screw driver and he popped off his trigger locks.