ValidateChildAttribute.cs

public class ValidateChildAttribute : ValidationAttribute {

    public override bool IsValid(object value) {
        if (value == null) return true;
        var isValid = IsItValid(value);
        if (!isValid) return false;
        if (value is IEnumerable) return ((IEnumerable)value).Cast<object>().All(IsItValid);
        return true;
    }

    private static bool IsItValid(object value) {
        return Validator.TryValidateObject(value, new ValidationContext(value, null, null), 
            new List<ValidationResult>(), true);
    }
}

This attribute allows the System.ComponentModel.DataAnnotations Validator.TryValidateObject method to validate into complex child properties of an object.