Access child members within parent class, C++

Question

I am facing a situation where I need to access child member variables inside the parent class. I know this is against OO principles but I have to deal with a scenario where

Answer 1

Would making an intermediate abstract class that contains the childMember you need to access be an option?

If so:

class Parent
{
    virtual void DoSomething() //Parent must be a polymorphing type to allow dyn_casting
    {
        if (AbstractChild* child = dynamic_cast(this)) {
            child->childMember = 0;
        } else //Its not an AbstractChild
    }
}

class AbstractChild : Parent 
{ 

     int childMember; 
     virtual void DoSomething() = 0;
}

class Child1 : AbstractChild {
     virtual void DoSomething() { }
}