Protruding coronal suture, skull x-ray. Cancer due to radiation?
I understand your concern.
First of all, though radiation in infancy is a known cause of childhood cancer, it is mostly therapeutic radiation (as in radiotherapy), which is implicated towards cancer.
I don't think the minute quantity of radiation from a skull Xray should cause any problems. So you should stop worrying about it and go ahead with what is the best thing the doctor suggests.