Like adults, children in Virtual Families can have serious illnesses (often discovered by use of the Doctor's Consultation) that can make everything seem normal other than the fact that they get sick often. This means that even though they might not be sick all the time, they might still have an illness.

As txmom2 says, "a bit weak" or "weak" (or even "extremely weak") is a good indication that something is wrong, especially if the house is stocked with plenty of food. Good luck!