There are some boxes on my app screens where users (kids) should type numbers in them by keyboard. Which cursor along with transition effect should we use to kids understand that they should type in it?
If you have a standard input, stick to a familiar cursor.
Even though they are not copying or pasting text as you indicated above, they are still placing the cursor in the text field (to prepare to enter a number) as a mouse option (correct me if I'm wrong)
Here's the MDN (Mozilla Developer Network) list of cursors:
The cursor change is helpful for when the mouse is entering the input area, but as user @ciammarino indicated below, without any mouse movements, is it clear to the user what is required for input?
We don't have a screenshot of your app, so it's hard to give specifics, but this is a consideration as well...