The 4 year old is a bit young, so this would be more difficult, however for the 10 year old this zhould work:
Show them the proper way to plug and unplug the tablet. Then explain that you do not replace items destroyed/lost by carelessness. If it simply wears out from use, that is fine. But if it is lost, dropped, cords pulled out wrong, etc., then you will not replace it.
Then stick to your guns.
I did this with my niece on stuff like this. When she ruined her mp3 player I bought her, she learned I was serious. She was much more careful with my future gifts - whereas she continued to be careless with her other belongings.
