I find that a strict routine that they play a strong role in is very important to keep them both entertained and prevent conflict. Give them some fun options for activities, and let them decide. I always like to make them think they are in control, even though they arent , that way they might me more willing to comply and co-operate. If there are issues, I would personally nip them in the bud right away and even talk to them infront of their parents about what has gone on during the day and whether this is acceptable behaviour. Perhaps set some goals with them and have a reward chart. Maybe ask mum how they are at home and what she does. Also I would make sure the parents are working hand in hand with you and are on the same page when it comes to discipline, and rules.