I suppose if you look back to your early childhood you accept everything people tell you, and that includes a heavy dose of irrationality - you’re told about tooth fairies and Father Christmas and things.

Dawkins is pointing to childhood as a period of epistemic dependence: young children largely trust authority figures and absorb cultural narratives without the critical filters adults can develop. By pairing benign myths (the tooth fairy, Santa) with “a heavy dose of irrationality,” he suggests that early habituation to believing comforting, unverifiable stories can normalize credulity more broadly. The remark fits his wider critique of faith and supernatural belief as culturally transmitted rather than evidence-based, and it underscores his emphasis on skepticism and scientific habits of mind as learned disciplines that must be cultivated against our early tendency to accept what we are told.