I'd like to build an sandbox for my toddler children. I'd like it to be inexpensive and low-maintenance (no repeated annual sealing of wood, etc.), and it seems that pressure-treated (as opposed to naturally rot-resistant) wood will have these qualities. But my research online is turning up conflicting opinions about safety issues - some say pressure treated wood contains harmful chemicals and others say it no longer does. Can someone help me decide, perhaps with references to authoritative sources?
Pressure treated wood is treated with chemicals. You have to ask yourself if there is any risk. Best strategy if you have any concern at all is to avoid the treated wood and go with something natural. You could use cedar and leave it unfinished use something like fir and paint it.
When I made a sandbox for kids years ago I used fir and painted it with safe paint. I also made the box with legs in the corners so that it was held up off the ground about 6 inches. Spaced fir planks for the bottom with landscape weed control fabric in the bottom allows for drainage whilst at the same time keeps the sand from escaping through the quarter inch spaces between the bottom planks.
I have raised garden beds made with natural redwood that are still structurally sound after 7 years. And these get watered every summer, hit with tillers, etc.
If it was me, I'd use redwood or cedar. Both will last long enough for a sandbox (especially if you cover it so it doesn't get waterlogged, and have decent drainage like @MichaelKaris suggested). One less nasty thing for your kids' bodies to deal with can't hurt.