Hi,
What do you mean by "save all bundled products with one click" ?
The onBeforeCartSave event is triggered when the cart of the user is modified (when a product is added, removed, updated, etc).
Are you saying that when a product is added to the cart, you want to also add to it the bundled products of that product to the cart ?
You'll find the products in $element->products
So you'll first want to loop on the products in there, and load the bundled products of these products with a MySQL query on the hikashop_product_related table. You'll then want to add them to $element->products.