What kind of small pets are we talking about? Lizards, frogs, snakes, mice and the like could all be captured and killed by a Flytrap or damage/kill a Flytrap during their struggles. In addition, as Tommyr mentioned, they require full sun, so placing them in a terrarium with your pets would not be conducive to the light levels these plants need.

You can certainly place several different types of carnivorous plants together, but keep the temperate species with other temperates and tropicals with tropicals, that way you can give them similar conditions year round as they need. If you plan to place Flytraps with anything, plant Drosera rotundifolia, Drosera intermedia, and Sarracenias together with them and use different levels of terrain, placing the Sarracenias lower so they are closer to the water level in the trays. Venus Flytraps and Drosera tend to prefer slightly higher elevations where they are not waterlogged. You can do that in a large pot or window planter. You do not need a terrarium at all, actually terrariums would be worse due to the chance of mold and bacterial infection, not to mention the inability to get good sun to the plants conveniently.