Don't sugarcoat or hide the truth from them. They will understand far more than you think.
So true. The fact that their grandparents belong to a cult can be explained in a simple way. Just try to use words appropriate for their age. Children as young as 8 can grasp the truth about Santa and the tooth fairy. They are better off knowing the facts. They are intelligent enough at around this age to differentiate the fact from the fiction. This is best done sooner rather than later. Postponing telling something they will find out anyway just prolongs the time frame during which they will feel deceived.
YC