Background and Aim: Bulk-fill composite resins were developed to reduce operation time through an easier application procedure. This could be of particular benefit in the field of paediatric dentistry. The aim of this in vitro study was to investigate the marginal integrity and the wear of eight bulk-fill materials in comparison to a compomer for restoring Class II restorations in primary molars after thermomechanical loading (TML). Design: Class II cavities in a total of 72 extracted primary molars were filled with eight different bulk-fill materials and one compomer, each in combination with a universal adhesive from the same manufacturer. The compomer served as an active control group. After 28 days of storage in distilled water at +37°C in an incubator, thermomechanical loading by thermocycling (2,500 cycles, +5°C/+55°C) and chewing simulation (100,000 cycles, 50N, 1.67Hz) followed. After storage, after thermocycling and after chewing simulation, impressions of the specimens were taken to produce replicas. Half of the replicas were used for a scanning electron microscopic evaluation in the form of a quantitative margin analysis, while the other half was scanned for the following wear analysis. For the statistical analysis, the Kruskal-Wallis test and the Wilcoxon signed-rank test were used (p < 0.05). Results: The results of the margin analysis showed a significant decrease in perfect margins after thermomechanical loading for all materials (p < 0.02). The proportion of marginal gaps increased significantly in all bulk-fill materials (p < 0.05), but not in the compomer (p > 0.05). The median proportion of marginal gaps after TML was between 14% (thermoviscous bulk-fill composite resin “VisCalor bulk”) and 80% (self-adhesive bulk-fill material “Surefil One”). Significant differences were detected between the tested filling materials with regard to their marginal integrity (Kruskal-Wallis test, p < 0.05). The thermoviscous bulk-fill composite resin “VisCalor bulk” exhibited significantly fewer marginal gaps and significantly higher proportions of perfect margins before and after TML than other filling materials. There were no significant differences in occlusal wear between the tested materials (p > 0.05). The average maximum vertical height loss varied between -60.58 µm and -134.72 µm. Conclusions: Due to the high proportion of perfect margins, the bulk-fill composite resin “VisCalor bulk” in combination with the universal adhesive “Futurabond U” should be examined within the scope of further studies, including testing under clinical conditions.