The answer to the above questions is: yes, it fits.
In Step 1 of this Instructable the unique features of the Rennrad 18 have been highlighted, in Step 2 the matching of the Rennrad 18 to a FollowMe Tandem is explained and in Step 3 some words are being spent on the license of this Instructable.
Step 1: Rennrad 18 by S'cool Kids Bikes
The Rennrad 18 is a high quality bike with 18-inch wheels, developed for children in the age of three to five years old as a run-and-bike learning system. The first challenge for this age group is to use the bike as a running bike, without the drive unit (i.e. the pedals and the chain) mounted. Next challenges (with pedals and chain mounted) are to use the pedals, the back-pedal brake and the hand brake.
You'll notice in the process of learning children to ride a bicycle that with the Rennrad 18 the stage of side-wheels (stabilisors) can be skipped, which is good as the use of side-wheels does not really challenge children to keep their balance.
Note that the Rennrad 18 is not intended for use on public roads, and thus that formally it shouldn't be used in tow by a FollowMe-Tandem on public roads.
The next step in this Instructable explains how to connect a Rennrad 18 to a FollowMe Tandem.
Step 2: Rennrad 18 on FollowMe-Tandem
This connection has been tested for the 'red' Rennrad 18 version which has a round vertical frame tube. The 'blue' version of the Rennrad 18 (i.e. an older version) has a square tube, which might involve even more filing of the clamp (not tested though).
The next step focuses on the CC-BY license applying to this Instructable.
Step 3: License
Republishing this Instructable is allowed, provided it is being attributed properly (cite the name openproducts, link to www.openproducts.org, www.instructables.com/member/openproducts, or the original Instructable. For other arrangements send a Private Message through the instructables member page (www.instructables.com/member/openproducts).
If this design infringes any rights then refer to Article 28 in the Terms of Service (www.instructables.com/tos.html).