〇 I agree that playing sports helps children become better people.
〇 I have two reasons to support my opinion.
First, Sports ordinary play with members, so Childrens can raise their teamwork and they can learn about cooperation with members.
→ First, sports usually involves playing with other members, so children can learn something about teamwork and cooperation with other people.(最初に、スポーツは通常、他のメンバーと一緒にプレーすることを伴うので、子供たちは他の人達とのチームワークや協力について学ぶことができます。)
〇 It is important for them to learn relationship with others.
Second, exercising by playing sports is good for Children healthy.
→ Second, playing sports is exercise good for children’s health. (2番目に、スポーツをすることは子供たちの健康に良い運動になります。)
△It is important for them to keep body healthy not only keep body healthy by eating healthy but also by playing sports.
→ It is important for them to keep their bodies healthy by not only eating balanced food but also playing sports.(子供たちにとって、バランスのいい食べ物を食べるだけではなく、スポーツをすることによっても身体を健康に保つことが大切です。)
△Therefore I think that they should play sports.
→ Therefore, I think that children should play sports.
※最後の文はまとめになるので、they(代名詞)ではなくchildren(名詞)を使う方が良いと思います。
参考にしてください。