You should remove the rocks and vacuum the tank. You should do a bigger volume of water changes each day than a bucket which is about three gallons cause five gallons weighs 45 pounds and you said this is an 8the grader? you should check that filter because it has begun to fail. If the tank was adequate for five months but 'suddenly' got dank and smelly, I'd say something is rotting in the tank (table scraps have been know to not get eaten and then...!) but that filter?
What kind of filter and is it functioning properly. Assuming the filter was adequate before (which I doubt) then why not now? There may be something in there rotting as well. I bet that there is a spot where a whole lot of nasty smelling things collects and poisons the tank. Maybe you can do a seriously major water change but in any case you'll have to clean everything. Not scrub away all the bacteria but be confident that there is no hidden pocket of bad stuff among the rocks or in the filter media etc.
This kind of situation needs activated charcoal or some other premium water purifier at least temporarily. It is likely that the tank needs a second large filter and better placement of the intakes to remove fish wastes more quickly and efficiently. Depending on the filter have them buy polyester floss to polish the water (remove suspended detritus and particulates that are presently passing through their filter and back into the tank) This relatively thick wad of floss needs be changed frequently but not the regular filter media so that this will preserve the cycle while 'sifting' the water. Get another filter because even when this problem if fixed the fish will need it.
It is doubtful that the water was changed as much as you were informed but maybe now it will be. Kids tend to feel nurturing by 'feeding' pets. If the water is cleaner and the tank bottom is clear (temporarily remove the rocks so that the kid can actually see the uneaten food everyday. then he'll start to believe you)
I'd aerate the water not so much for the fish but for the bacteria. Anything will help a little but the second filter should be first. A faster circulation flow more oxygen in the water will boost the fish immune system and good luck... it may be too late but I think it is too little maintenance over a period of a few months and that bill has come due.