There is a rising concern among parents that mobile advertisements may contain violent and sexual content even when the app itself is safe for children. Because mobile advertisements are not controlled by the content rating of apps, unexpected objectionable contents may occur and be harmful to children's mental health. This study is the first to explore the content appropriateness of the in-app advertisements on mobile devices from children's online safety perspective. We find in-app advertisements are common in the free apps designed for children on smart platforms. Experimental results show that a large percent of the in-app advertisements carry inappropriate contents for children. Unfortunately, neither mobile platforms nor advertising networks provide maturity policies to restrict the content appropriateness of the in-app advertisements. This research suggests that these challenges cannot easily be tackled by one entity. Instead, advertisement providers, advertising networks, app developers, and mobile platforms should collaborate in developing policies and mechanisms to monitor the content appropriateness of the in-app advertisements.