Hamsters, Guinea Pigs, Gerbils 

These members of the rodent family are inexpensive to purchase,  and quite cute, but the incidentals such as cages, food, and toys may be a bit costly.   

They can provide hours of fun for kids, but the main drawbacks are chewing and gnawing, and not being able to put males in the same cage with one another.   

They also have a short lifespan.  They require low maintenance as compared with a dog or cat, or dealing with the fish in a tank, but their cages should be cleaned regularly.  

Note: Gerbils have large incisors which means they tend to bite, so its important to provide them with baby carrots or other vegetables since they love to chew on just about anything. Hamsters and guinea pigs tend to be a bit less lively.