Why Colloquial Arabic?
The debate of formal vs. colloquial Arabic is heavier and deeper than a single blog post. We can refer you to many well studied, scientific articles about that (Dr. Riham Shindy provides excellent resources about the topic).
Let's address it simply, here.
We need books in Egyptian dialect as a stepping stone to children (and mothers) who are struggling with Arabic. We need attractive books that integrate Arabic in their daily life, that encourage them to read in Arabic without being intimidated by the richness of the language. We need to make Arabic important, cool, accessible so they can master it and feel confident enough to move onto formal Arabic.
To read more, please refer to the "useful links" section where you can see the research about this.