Yes fix a flat is poisonous if ingested and could cause death to dogs, cats and humans.
As long as you keep the fix a flat out of reach of children and pets and do not ingest it yourself then you should be just fine.
The fix a flat is a pretty safe product to use as long as you use the fix a flat as directed.
Fix a Flat is a great product to help fix flats when you have no spare or need to get off the road etc.
However when you do use fix a flat you still should get your tire fixed or replaced when possible.
And let the tire shop know you used some fix a flat so that they are prepared for the mess it creates.
Any product that could be toxic or poisonous should always be kept out of the reach of small children because they may try to eat it or drink it because they don't know any better.