How to render a block's child?

How to call child element as well in controller response?

I am trying to update cart where formkey and detete action is childhtml so how to get that childhtml?

Use the \Magento\Framework\View\Element\AbstractBlock::getChildHtml() method:

A usage example

A post was split to a new topic: How to get the form key and a child’s html of the checkout.cart.order.actions block?