I used to play Sailor Moon with my sister when I was a child.
I would play Sailor Moon with my sister when I was a child.
I and my sister used to pretend to be a Sailor Moon in our childhood.
"小さいときはよく妹とセーラームーンごっこをした"
小さいときは when I was a child, in our childhood, in my childhood
セーラームーンごっこ play Sailor Moon, pretend to be a Sailor Moon
～したものだった used to ~, would ~
I used to play Sailor Moon with my sister when I was a child.
I would play Sailor Moon with my sister when I was a child.
I and my sister used to pretend to be a Sailor Moon in our childhood.
When I was younger, I used to play Sailor Moon with my sister.
・「When I was younger, I used to play Sailor Moon with my sister.」
（意味)小さいときはよく妹とセーラームーンごっこをした。
<例文>When I was younger, I used to play Sailor Moon with my sister. We were so close.
<訳>小さいときはよく妹とセーラームーンごっこをした。とても仲良しでした。
ご参考になれば幸いです。
When I was little, I often played pretend Sailor Moon with my sister.
When I was little, I often played pretend Sailor Moon with my sister.
『小さいときはよく妹とセーラームーンごっこをした。』
この文章では "played pretend" を使って、妹さんとセーラームーンになりきって遊んだことを伝えています。
他の表現として：
My sister and I used to pretend to be Sailor Moon when we were kids. 『私たちは子供の頃、セーラームーンごっこをしていた。』
日本 
アメリカ合衆国 
