what kind of habitat did they live in?
There are conflicting reports that they preferred channels to floodplains.
Thescelosaurus
Conflicting reports have been made as to its preferred habitat; two papers suggest it preferred channels to floodplains, but another suggests it preferred the opposite. The possible preference for channels is based on the relative abundance of thescelosaur fossils in sandstones, 
what is thescelosaurus? It was a genus of small ornithopod dinosaur. when was it discovered? 1891 who found it? Paleontologists John Bell Hatcher and William H. Utterback
wiki:5919252
Thescelosaurus [SEP] Habitat
Conflicting reports have been made as to its preferred habitat; two papers suggest it preferred channels to floodplains, but another suggests it preferred the opposite. The possible preference for channels is based on the relative abundance of thescelosaur fossils in sandstones, representing channel environments, in comparison to mudstones, representing floodplain environments. No bonebeds or accumulations of multiple individuals have yet been reported. Dale Russell, in a popular work, noted that "Thescelosaurus" was the most common small herbivore in the Hell Creek Formation of the Fort Peck area. He described the environment of the time as a flat floodplain, with a relatively dry subtropical climate that supported a variety of plants ranging from angiosperm trees, to bald cypress, to ferns and ginkgos.