Can I buy a new iPad and legally send it to japan for grand child

Unless you have any export restrictions, you can purchase and send most things to other countries. What might actually matter to you is whether or not a warranty might be honoured should you have cause to make a claim…

Apple provides an international one-year limited warranty for products purchased in some regions. Warranty service is limited to the available service level in the country where the warranty claim is made.

AppleCare+ is only valid in the country of purchase.

Generally, you should aim to purchase a device in the country within which you intend to spend most time - as this often simplifies the warranty position should you ever need to call upon it.

https://store.apple.com/Catalog/Images/worldwidewarranty.html

Legal - iOS Warranty Document Us - Apple

You are asking end users here, not Apple employees or custom officials. There are likely import duties—and possibly restrictions— at the receiving end that only Japanese custom officials or import consultants can know.

The other issue is that, for the most part, Apple limits warranty service on iPad and iPhones to the country in which the device was purchased. From the pertinent Apple document covering the REGULAR warranty:

If your grandchild has an issue needing warranty service, the iPad would have to be shipped back to your home country, incurring more import/export fees and delays.

There are plenty of Apple Stores in Japan. From my point of view, it is easy to send the money and let your children do the shopping for your grand in Japan.

If it’s a Wi-Fi only iPad, there shouldn’t be any warranty issues.

if it’s a cellular iPad, it might cause problems.