Whether a child seat may be used depends on the country-specific legislation. See the topic 'Laws & regulations for S-Pedelecs'.
In principle, a suitable model can be mounted on the Stromer rack.
The ST7 driver 'David Jeanqaurtier' writes on Facebook:
The HAMAX Caress modelsthat are mounted on the frame bar (and not on the rack) are compatible with Stromer bikes. I have this model on my ST7, and with a few small protective films on the frame so as not to damage the bike, it is flawless.
