The old-fashioned language alone is enough to provide a hint that the characters are quoting something, and since you mention Paradise Lost, it's clear what they're quoting.
Strictly speaking, you can provide much less by way of attribution. For example:
If you want to imagine the future, imagine a boot... no, imagine a sneaker, laces trailing, kicking a pebble; imagine a stick, to poke at interesting things, and throw for a dog that may or may not decide to retrieve it; imagine a tuneless whistle, pounding some luckless popular song into insensibility; imagine a figure, half angel, half devil, all human...
Slouching hopefully towards Tadfield.
(Terry Pratchett and Neil Gaiman, Good Omens)
At no point does it actively attribute the reference to
If you want a picture of the future, imagine a boot stamping on a human face — forever.
(George Orwell, Nineteen Eighty-Four, part 3, chapter 3)
Or another example:
“Good night, sweet wizard,” Madeline purred, her hips grinding a slow rhythm against mine.
She drew the half-inch-wide mouth of the gun over my cheek as she took a slug of tequila and then rested the gun’s barrel gently on the spot she’d just kissed.
(Jim Butcher, Turn Coat, chapter 42)
No explicit mention of Shakespeare anywhere in the text.
It is quite common for literature to reference other literature. It's called 'intertextuality'. This is what wikipedia has to say on the issue of intertextuality vs. plagiarism:
As intertextuality, by definition, involves the (sometimes) purposeful use of other's work without proper citation, it is often mistaken for plagiarism. Plagiarism is the act of "using or closely imitating the language and thoughts of another author without authorization-". Whilst this does seem to include intertextuality, the intention and purpose of using of another's work, is what allows intertextuality to be excluded from this definition. When using intertextuality, it is usually a small excerpt of a hypotext that assists in the understanding of the new hypertext's original themes, characters or contexts. They use a part of another text and change its meaning by placing it in a different context. This means that they are using other's ideas to create or enhance their own new ideas, not simply plagiarising them. Intertextuality is based on the 'creation of new ideas', whilst plagiarism is often found in projects based on research to confirm your ideas. (source)