They are also quite perceptive. Often they are aware that something is going on when a parent is ill, despite the best efforts to protect them from the information. If children are not told the truth, their fantasies tend to run wild with imaginings worse than the reality. The revelation of a parent's HIV can come as a relief to children, making sense out of disjointed observations like medicine bottles, frequent doctor appointments, whispering on the phone, a parent's lack of energy, etc. Also, young children live in very self-centered worlds, believing they have magical power to cause or control events.
Life is hard.
HIV is part of it for children and for adults. There is no easy way to disclose, no best time, no ideal circumstance. The one guarantee is that a child will react with some painful and difficult feelings. The comfort to parents often comes in being able to be there for their children who must cope with some pretty grown-up stuff. Whether parents disclose earlier or later, ultimately the children will have to deal with the reality of the virus.