Switch to Switch – Transfer user with Breath of the Wild DLC Saves to new Switch

The content of the 3 chests :

• 1 Ruby
• 5x Bomb Arrows
• Nintendo Switch Shirt for Link to wear

That's what one player found, but as there's only the Switch Shirt which was announced, I think 'useful items' in the two others chests might also mean 'random useful items'.

Source

According to the Nintendo help pages:

While a console is active for your Nintendo Account, everyone on the console can play the games you’ve purchased.

Although this is talking about games, I can't imagine that DLC would work differently. One Reddit seems to confirm that DLC works just like the games:

...I can confirm that the dlc works on both accounts.

Numerous articles, such as this IGN one on the internet state that purchases made on the Switch are linked to the account. But what that means is, if you were to move you account to a new console, you can download anything you've previously purchased at no cost again, something that doesn't appear to be possible on previous Nintendo platforms.