Child Model Validation using Parent Model Values. Fluent Validation. MVC4

Create a custom property validator like this

public class AllChildBirtdaysMustBeLaterThanParent : PropertyValidator
{
    public AllChildBirtdaysMustBeLaterThanParent()
        : base("Property {PropertyName} contains children born before their parent!")
    {
    }

    protected override bool IsValid(PropertyValidatorContext context)
    {
        var parent = context.ParentContext.InstanceToValidate as Parent;
        var list = context.PropertyValue as IList<Child>;

        if (list != null)
        {
            return ! (list.Any(c => parent.BirthDay > c.BirthDay));
        }

        return true;
    }
}

Add rules like this

public class ParentValidator : AbstractValidator<Parent>
{
    public ParentValidator()
    {
        RuleFor(model => model.Name).NotEmpty();
        RuleFor(model => model.Children)
               .SetValidator(new AllChildBirtdaysMustBeLaterThanParent());

        // Collection validator
        RuleFor(model => model.Children).SetCollectionValidator(new ChildValidator());
    }
}

Alternative to the Custom Property validator is to use the Custom method:

    public ParentValidator()
    {
        RuleFor(model => model.Name).NotEmpty();
        RuleFor(model => model.Children).SetCollectionValidator(new ChildValidator());

        Custom(parent =>
        {
            if (parent.Children == null)
                return null;

            return parent.Children.Any(c => parent.BirthDay > c.BirthDay)
               ? new ValidationFailure("Children", "Child cannot be older than parent.")
               : null;
        });
    }

Crude way of showing indicies that failed: (should probably be name of some other identifier)

public class ParentValidator : AbstractValidator<Parent>
{
    public ParentValidator()
    {
        RuleFor(m => m.Children).SetCollectionValidator(new ChildValidator());

        Custom(parent =>
        {
            if (parent.Children == null)
                return null;

            var failIdx = parent.Children.Where(c => parent.BirthDay > c.BirthDay).Select(c => parent.Children.IndexOf(c));
            var failList = string.Join(",", failIdx);

            return failIdx.Count() > 0
               ? new ValidationFailure("Children", "Child cannot be older than parent. Fail on indicies " + failList)
               : null;
        });
    }

}