You can hide the timer permanently via the settings or on a per lesson basis by simply clicking on the time counter.
To hide the timer permanently, log into your parent dashboard and select Learner settings, under Settings.
Click the cog wheel beside the learner you want to remove the timer for. This will bring up their learner settings.
You can then hide the timer by toggling the Hide Lesson Timer box (When it's green, the timer is hidden.)