Solid foods should be introduced not later than the age of six months, regardless of whether the family adheres to a traditional, vegetarian, or vegan diet. The aim of this study was to compare the knowledge on the complementary feeding of infants older than six months among mothers who adhere to traditional and vegetarian diets and to identify problems that require the assistance of a dietician. A total of 251 mothers of children aged 10–12 months participated in the study. Only 10% of vegetarian mothers declared that they had placed their children on a vegetarian diet, whereas 36 mothers adhered to a lacto-ovo-vegetarian diet during complementary feeding in the first year of life. Mothers adhering to a traditional diet were characterized by lower levels of knowledge (4.1 ± 2.3 points) than vegetarian mothers (5.3 ± 2.1 points). Mothers following a traditional diet were more likely to indicate a higher than recommended number of daily meals (OR = 1.76; Cl: 1.31–1.97, p < 0.001). Significant differences were noted in the respondents’ adherence to the BLW method, which was more readily implemented by vegan (p < 0.05) and lacto-vegetarian (p < 0.05) mothers and was least popular among mothers following a traditional diet (OR = 0.81; CI: 0.66–1.23, p < 0.04). Vegetarian mothers unnecessarily delayed the introduction of gluten and potentially allergizing foods to the children’s diets, sometimes even past the age of 12 months.