A fish in the wild can endure a little flux in salt or temp if a current sweeps through, or an upwelling, etc, not usually a big deal. The salinity threat is to the kidneys, which have to get the fish to excrete and thus intake water to get the body balanced for the ambient salinity. The kidneys can work only so fast, and if the fish's cells are at one salinity and the ocean at another, it could cause cell rupture---but a fish has a lot of exposed surface to 'sweat' out or absorb salt. So it has some trouble adjusting, but given minimal change, minimal trouble. A point of salinity is not a big problem. 3 points, that's approaching a problem...maybe fatality.
A little fish actually adjusts to a heat situation faster than a big fish---just a body mass issue; but when dealing with cold rather than heat getting to the body core, this can be a bit stressful.
Worst case is salinity for inverts: snails and such can't 'sweat' and have very, very limited ability to intake and expel salt---they're not as fast to adjust as fish are, because they have only one avenue for getting the inner flesh adjusted to the new salinity. As for temperature I'm sure their shells serve as a temperature buffer, slower to heat up and cool down, but salinity difference causes what is called 'osmotic shock,' as in osmosis, the process of absorbing fluid through a membrane, meaning they have one salinity inside them and the ocean is something else: result, kidney damage and death within a few days as body toxins build up and can't be cleared by damaged kidneys.
This is what I understand to be the case. Hope it clarifies.
If your son is under age 6, do discourage him from the tank area: children absolutely love to 'feed' things, and they tend to imitate/play at what they see when the notion and the opportunity hit. They can overfeed your tank by dumping in half a can of food, decide to clean your tank with dishwashing detergent, decide their toy truck would look interesting in it---some are fascinated with tv remotes as floatation devices; and the addition of half a jar of dry calcium isn't even to be contemplated. The future does not occur to the under-six,

except as re the wrath of mommy/daddy and the concept of 'mine' vs. 'daddy's'. Above 6, they're perfectly capable of caring for a small freshwater tank of their own.
And page 2 is mine.