Aim of this study was to investigate at what age German children master prosodic and morphological constraints in the acquisition of the word formation paradigm -heit/-keit, which is comparable to English -ness, and whether children with Specific Language Impairment (SLI) have difficulties identifying the prosodic cues from the input. Derived words with -heit contain simple bases with final stress and those with -keit have complex bases with a weak final syllable. Three groups of typically developing children (four, six and eight years old) and 18 children with SLI (from 8 to 10 years) had to produce either -heit or -keit derivations in a sentence completion task. The results show that typically developing children mastered these derivations by the age of six only when both prosodic and morphological cues were present, while eight-year-old children performed almost adult-like. In contrast, most children with SLI did not produce systematic responses that follow prosodic and/or morphological constraints. The findings support the assumption that children with SLI are less sensitive to prosodic properties of grammatical forms than typically developing peers.