Do you think it is important for children to play sports?

以下、回答。
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I think it is important for children to play sports.

There are two reasons for this.

First, playing sports is healthy.

When they run outside, they become strong.

Second, they can make friends through sports.

When they play sports with friends, they can become to know each other.

For these reasons above, I believe it is important for children to play sports.
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補足説明
①以下の表現もOK。
I believe it is essential for children to play sports.
In my opinion, it is vital for children to play sports.

②以下の表現もOK。
I have two reasons to support my opinion.
There are two reasons as shown below.

③以下の表現もOK。
It is healthy to play sports.

④以下の表現もOK。
Second, playing sports provide a chance for them to make friends.

⑤以下の表現もOK。
they can strengthen their relationship


1) above をつけて、以上の理由から、という表現も出来ます。
2) important を言い換えて、essential や vital に書き換えても表現出来ます。