I remember a study how glioblastoma's were caused by extensive dental X-rays in young children. These manifested in brain cancers decades later.

Don't listen to all the banana cancer radiation comparisons. True studies have shown that for every increase in radiation that you're exposed to there's a proportional increase in cancer. So small amounts of radiation including even dental X-rays increase your risk of cancer by a similar small amount. But extensive full skull around the head dental X-rays to detect early cavities in teeth that are going to fall out anyway are absolutely extraordinarily irresponsible. Let your kid get the cavity and have it filled when they get the cavity if they need to. Do not let a dentist fill your kids mouth and head with radiation for something as stupid as that.