Diazinon is a very lethal substance. Depending on where you use it (indoors or outdoors) and in what concentrations (different concentrations for different 'pests'), the answer to your question is, basically, it depends.
I'm not going to go into any details on the 'depending' because you need to read up on, specifically, what you should do depending on the 'depends'. I suggest you search the WEB like this: Go to AltaVista WEB Search and enter in: +diazinon & +pets & +water EXACTLY as you see I've bolded. Then press Search. You'll get a plethora of info on Diazinon, its cautions on pets, and its cautions on water supplies, AND its effectiveness on household pests. And you'll get more info than you want but it's important to get good info.