Undo Factory Reset on oneplus
Many Android phones can be booted into a recovery mode and factory reset from there. In the case of the 6T you power it off then hold down volume up at the same time as pressing the power button. That boots it into recovery mode, and from there the phone can be reset by selecting the appropriate option. Your child must have managed this without meaning to.

There isn't any way of retrieving the data from the phone. OnePlus or the FBI might be able to do it but there's no way for you or I to do it. However if you go back through the setup and choose the option to restore your data from Google you will get back anything that has been backed up to the cloud.

Reading the data from a factory reset phone would be a serious security flaw. It would mean anyone who stole your phone could reset it to wipe your phone lock then read your data off the phone.