The definition of sleep bruxism (SB) has changed over the years, and although it is no longer defined as a disorder, it is considered a risk factor that can result in prejudicial systemic and stomatognathic issues. The prevalence of SB in children is variable among studies, and its decrease during adolescence is a controversial matter among authors. We aimed to determine SB prevalence and assess its trend with age in a sample of pediatric patients who frequented the Lisbon Pediatric Dentistry Service. We conducted a retrospective observational study based on information provided by patients’ parents/caregivers and without examination or polysomnography examination. Data were collected between August 2019 and December 2020 by five dentists. A total of 1900 patients were included, 50.6% and 49.4% of which were male and female, respectively. Of the total sample, 334 (17.6%, 95% confidence interval of 15.9–19.4%) had bruxism, corresponding to 18.9% and 16.2% of male and female patients, respectively (p = 0.121). SB was reported in 20.7% of patients 0–6 years old, in 19.4% of those 7–11 years old, and in 14.6% of those 12–17 years old. In conclusion, SB showed a slightly higher prevalence among male pediatric patients, but further studies are needed to rule out confounding factors.