In a recent study concerning children and media consumption, researchers discovered that the majority of apps targeted at children 5 years of age and younger contained advertising. Some of these ads were not clearly marked, were difficult to close, or contained disturbing imagery. Additionally, some of the ads pushed the user to make purchases, a clear violation of the FTCs rules regarding ads used in children's media.
https://www.notebookcheck.net/Apps-marketed-at-young-children-have-a-major-ad-problem.354066.0.html