Passing variables to a child html block in Magento 2

You have to define public function setProduct in your block class:

public function setProduct($product)
{
    $this->product = $product;
}

and then you will be able to get it in your block :)

Also, use getChildBlock function instead of getChild

Now you have to use your newly created function, setProduct($_product) instead of setData('product', $_product').

Use below code:

$block->getChildBlock("offer_list")->setData("product", $_product);

to

$this->getLayout()->createBlock('PR\Catalog\Block\Product\Offers')->setProduct($_product);

In your block class add the following:

protected $product;

public function setProduct($_product){
    $this->product = $_product;
}

public function getProduct(){
    return $this->product;
}

In the template file that you call the child block do it like this:

<?php /* Start Custom Changes */ ?>
<?php if ($myBlock = $block->getChildBlock('my_child_block')): ?>
    <?php
        $myBlock->setProduct($_product); 
        echo $block->getChildHtml('my_child_block', false);
    ?>
<?php endif; ?>
<?php /* End Custom Changes */ ?>

Now in your child block template file:

<?php echo $block->getProduct()->getId(); ?>