Live Sand for the bottom. Live rock will also be greatly beneficial, though it won't be necessary. You will want/need about a pound of sand for every gallon of tank, maybe a little less. The instant ocean is for making your own salt water. If you getting sw from the shop then you won't need it. Do not, DO NOT go with an undergravel filter, your best bet will be a wet/dry filter. Depending on the size of the tank this may not be worthwhile and a good hang-on or canister filter(preferable) will work. If you go the canister route, I strongly suggest an Eheim, do not go with a Magnum it really can't support the biological filtration, no matter what anyone at the locat pet store says. As for hang-on I would consider a Millenium (made by marineland).
If you guys aren't very experienced you/your son may want to start reading at <a href="http://www.thekrib.com/">the Krib.com</a> under the marine section. Lots of info.
As for everyone LOL about the freshwater fishies...
My 27-28 inch arrowana and roughly the same size Pseudodoras Niger. The tank is a 72"x24'x24' 180gallons to get a better idea of how big the fish are.
<img src="http://www.zjstech.net/~library/7804/Fish/arrow1.jpg">
<img src="http://www.zjstech.net/~library/7804/Fish/180-1.jpg">