Always give exposure to the kids about the language as much as possible. It can be done by using textbooks and apps.
Their time should be filled with fun activities for the child, which expose them to the language, which is things like switching storybooks to bilingual ones with the Arabic language. You can also include, playing Arabic games, watching Arabic cartoons, etc.
1. Use wordbooks
You need to teach them how to use these words in basic phrases and simple sentences. In the books like Arabiyyatu Bayna Yadayk, you can make plenty of simple sentences for your kids. Books like these are a great option.
Also, have Qaeda al-Noorani, a book that will teach them how to read the Quran along with the Tajweed. Once they start to read, focus on building vocabulary.
2. Switch cartoons to Arabic
Kids spend their significant time watching cartoons. Thus, switch all the cartoons or other channels to Arabic. Most cartoons are in Fusha or Formal Arabic, check the resources section for a curated list of cartoons in Arabic.
3. Make them read storybooks
Another option is to make them read storybooks in the Arabic language. Search for bilingual books with stories in both Arabic and English, so you can explain them easily. Always select visually pleasing books that children will like reading repeatedly, and these books are always a great investment.