When you use the term "router" I am not 100% sure if you mean MX240 or QFX5200 -;( Both can equally route.
I am going to somewhat assume you have native 10GE interfaces on the MX240. If you send a show chassis hardware and tell us what FPC/slot(s) you plan to use, can tell from that.
Where you need the break-out cable woud be on the QFX5200-32C, as its interfaces are native 100GE, not 10GE. So you'll need 2 things. One is some sort of break-out cable and second a channelized configuration. Unfortuantely when I went to look for the docs, I found this link:
Unfortuantely the associated hardware quide is no real help, and the channelized interfaces section does not talk about the QFX5200. Maybe you can go to this link and hit the feedback button at the top and tell Juniper that this documentation is not sufficient to asnswer your question.
Now as RONF responded you will need some form of break-out cable on the QFX5200 side. For this go to Support Site -> Pathfinder -> Hardware Compatibility Guide and then find product QFX5200. This gets you to;
https://pathfinder.juniper.net/hct/product/#prd=QFX5200 where you can click on QSFPP-4X10GE which is the one break-out cable supported. You find details here:
https://pathfinder.juniper.net/hct/model/?component=QSFPP-4X10GE-SR&prd=QFX5200&cat=Transceivers
Hope this helps and good luck.