For minors, the primary travel document is a passport (from age 14). A birth certificate is only required to prove parenthood if surnames differ or the child is not yet 14. Train travel booking uses the passport/birth certificate details, and border guards rarely deny boarding unless there are specific legal restrictions (e.g., FSSP debts, though this varies by case). Official rules usually do not require extra 'parental consent' documents unless a child is traveling with a third-party coach.
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