Cranial osteopathy is modern quackery, I'm afraid. It's looked down on even among osteopaths. One osteopath wrote:
"This treatment regime lacks a biologically plausible mechanism, shows no diagnostic reliability, and offers little hope that any direct clinical effect will ever be shown. In spite of almost uniformly negative research findings, "cranial" methods remain popular with many practitioners and patients."
I think you will find most serious scientific reviews of it come to the same conclusion: "at present, there is insufficient evidence to support cranial osteopathy as being relevant for the diagnosis or treatment of patients."
So:
What it is, how it benefits, any unwanted side effects etc
What it is: a belief that spinal fluid pulsates and that this affects the body. Cranial osteopaths are meant to be able to detect the pulsation through skilled touch of the skull and alter this through massage. Basically it is a gentle massage of the head.
How it benefits: supposedly through altering pulsating fluid and the human skull. Really it is a mix of the placebo effect and quite a nice relaxing massage.
Unwanted side effects: it's a gentle baby massage, there shouldn't be any. But a practitioner who makes mistake and pushes too hard can of course hurt a baby's soft skull.
Would you recommend? No, it is expensive. It has no scientifically proven benefits. It is modern snake oil. It certainly doesn't work by changing electrical pulsations in your baby's brain and spine.
HOWEVER gently massaging any baby may calm them down - so instead of paying an "alternative practitioner" to alter your baby's "skull pulsations" you can try some baby massage at home yourself for free.