Hamsters sleep in the day and come out at night. Unless you have one of those night nurseries for insomniac children, what's the point?!
Fish are pretty and easy to look after if you spend a bit on a nice filter, but are rubbish when you take them for walks: They flap about then die. Best left in water.
Gerbils sleep 2 hours then wake up and are interesting for 2 hours. They are very fast and hard to hold, but at least they eat their own babies. They are also organic shredders and will destroy a phone book in 30 seconds of vicious biting frenzy.
Degus: Message to Degus everywhere. You're gerbils! Get over it!
Tortoises? Rocks with legs. Boring, but pretty much stay the same when they die.
Giant Snails sound interesting but are really not. Few staff like them so they stay in their tank looking like a lot of big snails in a tank.
Goats/donkeys/ponys need lots of looking after preferably by women in jodhpurs with strangely uncontrollable hair with bits of straw in. Boy versions of these animals will lead to a lot of awkward moments.
Rabbits are cuddly and edible. Win-win, but some children don't like eating pets. Weird huh?
Guinea pigs are not even real pigs. They don't hop or have long ears so are basically rabbits without the interesting bits.
Snakes are cool. Alice Cooper has some. They freak out some people which is funny.
Cats and dogs are not nursery pets.
Insects: Don't be weird.
Birds poo everywhere and escape. Some can be taught very naughty words so be careful if you get a second hand one.