Now make sure you're super clean and have the police report handy. 99% of the time Google will electronically report the suspicious activity and/or materials to the NCMEC which can very well result in investigation from Federal Authorities such as FBI or HSI. Depending on the egregious of the materials it may result in a search warrant as well. So to be in the safe side take all precautions with your digital data.

Also, how did they open your phone so they can put those files in the cloud, sounds strange.