Is a creator of beneficial educational content for children on smart devices held sinful if a child is exposed to harmful online content due to possessing the device, motivated by the desire to benefit from that content?

There is no objection to producing educational materials that are free from objectionable content, and there is no sin upon the producer if the child accesses forbidden websites. The responsibility lies with the parents to monitor the child, for if a child uses any permissible or beneficial things in a forbidden or harmful way, the responsibility for that falls upon the parents.