Buying Request on Itunes

Hi Everyone , my son has his own iTunes account, whenever he wants to purchase something I get a request that I accept or refuse. Strangely enough, I do not get the request on my iPhone or my MacBook Air. I do receive it only on my iMac. Why doesn't it work on the other devices ? Is there some setting I must change on my iTunes , and if so where and how ?

Thanks for your help !