Giant African Land Snails (GALS) are tropical snails, there are many species and sub-species of GALS but probably the most common found snail in the UK and probably a good starting point is the Achatina Fulica, this is a relatively large snail, growing to around 10-16cm in shell length.
They require minimal attention to survive quite happily, and have a very reasonable average life span of 3 - 7 years in captivity, in the wild this can almost double, so as a child's pet you should be prepared to offer it a home for the foreseeable future.
I have found my snails to be sociable animals and they prefer the company of other snails, being much more active when kept with other snails than when kept in solitude, however one important thing to remember is that snails will mate quite easily, and if left un-checked you will soon find yourself over run with baby snails (see "Breeding your snail" for more information).
Your snails home should be large enough to house adult snails of the species you have chosen to keep, it should have a secure lid and ventilation, snails are also spatially aware, so their environment should have some detail to it, areas for them to explore, hide away in and provide them with a something to 
explore, otherwise they can become bored and inactive, there should be a soil base to the tank, there are many types of substrate base you can consider and these will be covered in more detail in "housing your snail".
Very basically you will need to provide your snail with a clean, damp (not sodden) environment, that in winter months may require additional heating in the form of heat mats, a daily supply of fresh, clean pesticide free food, a source of calcium (important for good health and shell growth, normally cuttlefish bone). you will need to clean the tank on a regular bi-weekly basis, and remove waste food/deposits on a daily basis. and a shallow water dish to provide water for them to drink and bathe in, this will also help with humidity in the tank, spraying the tank on a daily basis with a plant sprayer/mister is also strongly recommended.
Overall these are a simple pet to keep, that require minimal care, their home is easy to set up and maintain, and will on average outlive the normal hamster or gerbil pet.