Seat reservations on the TGV in Germany are optional so unless someone else has made a reservation for your seats you can stay in them. But, as there are 5 of you I would make seat reservations from Karlsruhe to München. If you have a child under 15 in your group you get a discount on the seat reservations if you maje the reservation through DB.

Depending on where you made your reservation, you might be able to cancel for free and rebook it as Paris-Munich (but do make sure there is sufficient availability better you do that! The cancelled seats aren't guaranteed to become available again).

Otherwise, you can stay in your seats and take the risk that nobody has reserved them.

If you do reserve Karlsruhe-Munich seats on the DB website, check if you can select then from a seat map - I don't know if they offer this for the TGVs.

Unfortunately, they don’t...

Thanks for the replies. In the end I rebooked the seat reservation Paris / Munich and then claimed a refund for the Paris / Karlsruhe original. It cost me the booking fee (10 Euro), but I guess making onward seat reservations would have cost me more than that anyway.

If you only paid 10 EUR for Paris-Karlsruhe, then that was not a seat reservation that is valid with Interrail/Eurail. The correct cost for a seat reservation on an ICE-train between France and Germany is 19 EUR per person. If the reservation is made through DB and if there are children under 15 in the group, there's a family discount.

How much did you pay for the new reservation?

^^I’m not sure if ​@NMcG meant a service fee for refunding his reservations.

If he bought his reservation through eurail: 5 x 2€  = 10€

Sorry - my message was misleading. 

Yes: I paid the 19EUR per person for the pass holder reservation. But I was refunded the original cost minus a 10EUR service fee. So the net cost to me for extending the reservations was only 10EUR.

Thanks for flagging this though - I should've been clearer.