I'm trying to figure it out for couple of days, google best ux practices for building apps for kids, but none of them says "you should make your app in landscape mode". Tried to search for answers here, but the only thing I have found is that kids seem to prefer positioning devices in landscape mode. Why is that?
I'd say because landscape is likely more appropriate for the content displayed in most apps for kids.
Portrait Mode is generally used to display lists of things, or long text passages, because you can fit many items onto the screen. For example, when browsing Amazon in portrait mode, I can fit ~5 items onto the screen, whereas in landscape mode, I can fit only ~3 items on the screen. In general, most things "adults" do either revolve around lists (social media, shopping, e-mail, etc.) or text (Stack Exchange, Wikipedia, etc.).
Landscape Mode is generally used to display more "visual" or "interactive" content. Most applications for kids are usually going to be interactive or try to present content in a visually engaging way, rather than trying to achieve high information density, and cramming the most information into the smallest space.
Landscape Mode may be seen as "more engaging", because it better matches our natural "screen ratio" of being wider than tall.
As a result, it doesn't make sense to say "Apps for Kids should be in Landscape Mode", because that's simply not true. It's just that Landscape Mode is generally the better choice for the kinds of application marketed towards kids.
I assume they may have had a few of these considerations for this.
Most video games are in landscape mode as compared to portrait mode, even in mobile. Since kids are already playing mobile games they are pretty used to landscape mode than portrait.
As kid's arms and hands may not be stronger as compared to adults, making the application landscape forces the kids to hold the phone with both hands, reducing the likelihood of dropping it.
Another positive might be that it's easier for people to walk around while using an app in portrait mode. This can be hazardous especially if the app is more immersive. This can make kids more prone to accidents. When the app is in landscape mode users are less likely to use it while walking or multitasking as holding it takes both hands.