Secondhand smoke exposure (SHS) causes a disproportionate health burden for children, yet existing smoke-free laws are often poorly enforced. We monitored air quality while observing children and adult nonsmokers present in public venues during Ramadan, a period of Muslim religious observance marked by family and social gatherings, in Jakarta, Indonesia. A repeated-measures design was used to assess indoor air quality during and after Ramadan in 43 restaurants and in five smoke-free control venues. Fine particulate matter of 2.5 microns or less (PM2.5) was sampled. The average number of children and active smokers present in each venue was also observed. PM2.5 levels were significantly higher during Ramadan (mean 86.5 µg/m3) compared with post-Ramadan (mean 63.2 µg/m3) in smoking venues (p = 0.015). During Ramadan, there were more active smokers (p = 0.012) and children (p = 0.051) observed in venues where smoking occurred, compared with the same venues post-Ramadan. Poor enforcement of the smoke-free law in Jakarta has failed to protect children from SHS exposure in public venues during Ramadan. Collaboration between the government, NGOs (such as the Indonesian Cancer Foundation (YKI) and the Smoking Control Foundation (LM3)), religious leaders, and venue owners and managers must be developed to ensure that the comprehensive smoking bans apply to all venues, and that smoke-free laws are enforced.