Some of the items that you buy aren't useable until the tamagotchi is an adult. Usually, this is around 2-4 years of age, depending on the tamagotchi.
If you go to "items" and then click "b" on the item you want to use, but all it does is give a beep, your tamagotchi will probably just have to wait until s/he's an adult. I know that you can use things like the building blocks when you're a child, but things like the costumes and music aren't useable until the tamagotchi has matured and becomes an adult.
Hope that answers your question.
