The way inheritance works for Splashtop is that the parent policy pushes its state down the policy tree, so if you want to enable Splashtop for a child policy, you must also enable it for the parent. In other words, Splashtop must be enabled per policy where you'd like to use the tool.
Disabling the integration in the child policy will override the state of the parent. Thus, enabling the integration at the parent level does not automatically enable it in child policies. Child policies have Splashtop disabled by default.
If the Flex Policies feature is enabled in your tenant, assigning a device to at least 1 policy where Splashtop is enabled will allow Splashtop to be deployed to the device.