Some snakes are more tolerant than others so you really have to watch for signs of stress from the snake. If it's always trying to get away then it's probably getting too stressed. If it stops eating then don't handle it until it is eating consistently. Of course don't handle it for 24 to 48 hours after it has eaten.
Older snakes are usually more tolerant than younger ones. As the snake gets older they can probably handle it for longer periods of time.
Unfortunately, the answer to how long you should let the kids handle the snakes is...Not as long as they want to. Children that young might want to play with the snakes much longer than is good for the snakes.
It sounds like you are teaching the kids that they can only handle the snakes for a short while because it could hurt the snakes if they are out for too long. Once the kids understand that...they will good little herpers.
If there is anything the nonconformist hates worse than a conformist, it's another nonconformist who doesn't conform to the prevailing standard of nonconformity.