「子どもと一緒にいるだけで、ダイエットになる」= just being with my child is like being on a diet / I stay slim by running around after my 2-year-old / Looking after my toddler keeps me fit
ボキャブラリー
being with my child = 子供と一緒にいる事
like = のように、みたい
be on a diet = ダイエットする
stay slim = スリムなままでいる
running around after = (子供と)付き合う(イディオムのような表現です)
2-year-old = 2歳の子供
look after = 面倒を見る
toddler = 小さい子供 (1歳~3歳くらいの小児)
keep 人 fit = 体調を保つ、健康を保つ