Without further context, "his child was in an accident" suggests a child of ~1-12/16/18. (Younger than ~1 and we should say "baby".) "His son was in an acciden

Without further context, "his child was in an accident" suggests a child of ~1-12/16/18. (Younger than ~1 and we should say "baby".) "His son was in an accident" could equally refer to an adult.