"Children are half price, 50 yen. Two children's tickets, please."
"Children are half price, 50 yen. Two children's tickets, please."
"Kids are half price, 50 yen. Two kids' tickets, please."
"A child ticket is 50 yen. Can I have two child tickets, please?"
「子どもは大人の半額の50円です」「子ども二枚下さい」は、
・半額 → half price
・子ども → children / kids
・チケット(切符) → ticket
・二枚下さい → two tickets, please
この4つを組み合わせると自然な英語になります。