Today I took the day off because of my child's seven-five-three festival.
ご質問ありがとうございます。
「今日は子供の七五三で会社を休みます」は英語で「Today I took the day off because of my child's seven-five-three festival.」と言えます。
seven-five-three festivalに関して、説明したいなら、長い和文も英訳しましょう。
下記の英文の通りになります。
Today I took the day off because of my daughter's seven-five-three festival. This festival is celebrated in Japan for the children who are turning 7, 5, or 3 years old to wish for their health and growth. We go to a shrine and later go to a photo studio to take family photos.
My eldest daughter is 3 years old and my other child is 1 year and 11 months so I would like to celebrate for both of them together. It's usual for the children to wear a kimono for this event but I'm a bit worried because they are just small children and who knows how it'll go at the shrine visit and photo shoot.
ご参考になれば幸いです。
Today is my child’s shichigosan so I will be taking the day off.
「今日は子供の七五三で会社を休みます」
- Today is y child’s shichigosan so I will be taking the day off.
七五三は英語に訳せませんのでそのままで良いと思います。
Taking the day off = 今日休みます//一日休みます
「今日は子供の七五三のために会社を休みます。
Today is my child’s shichigosan so I will be taking the day off.
七五三は７歳と５歳と３歳になった子供の健やかな成長を祝う日本の文化で、神社にお参りに行き、そのあと写真館で家族写真を撮ります。
It is a Japanese tradition to celebrate the healthy growth of children aged 7, 5, and 3 by going to a shrine and taking a family photo at a studio afterwards.
上の子が３歳で、下の子はまだ１歳11か月なのですが、一緒に３歳のお祝いをしてしまいます。
My oldest child is 3 years old and the younger one is only a year and 11 months but they will celebrate too.
着物を着せるのですが、子供たちがまだ小さいので、着付けやお参り、写真撮影までうまくできるか心配です」
We will make them put on a kimono but the kids are still small so I’m worried if we will be able to safely dress them, pay respects at the shrine, and take pictures.
I'm taking the day off for my child's Shichi-Go-San celebration.
「今日は子供の七五三で会社を休みます」は、I'm taking the day off for my child's Shichi-Go-San celebration. と表現できます。
Shichi-Go-San は日本の固有の文化なので、そのままの呼び名を使うと伝わりやすいです。take the day off は「仕事を休む」という定番のフレーズです。
アメリカ合衆国 
