My PILs have gone on holiday for 4 weeks (went last Friday) and we just can't get it through to our 5 yo DS1 (who is quite close to them, FIL in particular) that they have gone on holiday! He keeps asking for Granda to pick him up and asks if he can sleep at their house and we give him the same answer each time, that they are on holiday and he can see them when they come back.
Any advice on how to get it through to him?