Here are my questions:
Is it ok to leave the developer, stop, and fixer in their trays for days, weeks, (or even months?) -- having simply covered them with tin foil?
Probably not. aside from the danger of having open trays around the house, the chemicals will most likely oxidize and lose their spunk.
Will they "spoil" sitting like this? (The pantry stays between 60-80 degrees (F) depending on the day).
Over time, yes. And the dev and stop will probably stain your trays.
Of course I am concerned about these chemicals hanging out in the pantry where we keep the Cheerios, cooking ingrediants, etc. as I can smell the vinegary stop bath. Is having them sit out in these trays a hazard to my health?
This is really a bad idea. Don't use or leave chemicals around food unless you like getting sick.
Should I never do this and always put them in my storage jugs (most likely well-cleaned plastic milk jugs as I am still poor)?
Most people I know use developer as a one shot, and save the stop and fix in jugs. I use everything one shot because I am lazy.
All thoughts would be highly appreciated as I want to have children someday and don't want my expensive chemicals going bad.
Not to sound overly harsh, but expensive chemicals are the least of your worries. Chemicals in trays around food + kids is really really bad.
On a second topic, how much use do you typically get from a fresh batch of developer, stop bath, and fixer (your run-of-the-mill Kodak stuff) in an 8x10 tray?
More than I can print in one session.
About how many 8x10 prints could these chemicals handle if you were using 8 x 10 trays filled with 1/2 gallon?
Is 1/2 gallon too much to fill your 8x10 trays with?
I use one liter per 8x10 tray.
Do chemicals get weaker over time in storage?
Told you they were basic questions, but any thoughts or direction to other sources would be greatly appreciated!