I guess that ant farms are pretty standard fare for young kids, so what can you grow and observe if your kid (or you!) wants something different? You can always try growing some frogs!
Touting an environment similar to a rainforest in a box, this live frog rearing kit lets you grow and observe how a tadpole morphs into a frog. The leopard frog is shipped to you as a live tadpole which you and your kids place into the environment to watch it grow.
The end result is pretty cool and similar to a full-blown vivarium, but more kid friendly. Get your very own Live Frog Rearing Kit for only $30. A cheaper, but much less interesting version, can be had for $23.