Hi Tombliboo, sorry it took a while to get back to you!
Unfortunately as Skylark mentioned syncing playback data isn't possible at the moment. This is something we do have in mind, but it won't be implemented any time soon.
In the meantime, you may try Skylark's suggestion. You may also add a note on your phone calendar or in your (physical) planner / journal, or wherever it's most convenient for you.
Do also note that you can manually mark your lessons as "played" (i.e., the circle icon beside each Course folder or lesson):
1. Go to Little Reader's Play Courses or Play & Edit > Courses screen
2. Right-click on the lesson your child has finished at Grandma's and select "Mark Played" (the circle icon will then become "shaded" or turn blue)
If you need to mark several lessons at a time, it's best to do that from the Play & Edit > Courses screen -- just select all the folders or lessons and do Step 2 above.