I've got a a 4-year-old who's in school, and is quite big for their age. I've just booked a train journey, paying only for myself and relying on the fact that under 5s travel free (though quite possibly without a seat).
Is there ever a need to prove that the child is under 5 when relying on the free ticket (and if so how)? I'd rather not have to carry their birth certificate as it's a valuable document that won't fit in my wallet and travelling by train with a small person (and all their stuff) isn't easy without another valuable item to worry about. On the other hand I don't want any awkwardness from an overzealous official. I haven't been able to find anything obvious on the train companies' websites.