Let me quickly reprise how forms to publish child posts can be automatically linked to parent posts.
This pre-supposes that you have created a one-to-many (i.e. parent << child ) relationship, and that the starting point for publishing the child post is the parent post (so that the context determines which parent post is to be linked to the new child post).
It can happen in one of two ways.
The first is where from the parent post you add a link to a form to publish the child post. The link will contain a URL parameter that specifies the ID of the parent post. The child form includes a selector for the parent, and it "listens" for that URL parameter: if present it will set the parent selector to the post whose ID is passed in the URL parameter. That's how the child post knows which parent it should be connected to.
To set this up you make your form to publish a child post. It will automatically include the parent selector (which you may want to hide with CSS). You add this form to a page. Then you edit your template for parent posts and where you want the link to appear you use the Toolset Forms button to insert the link (see screenshot for the resulting dialog).
The second way is where instead of displaying a link to add a child post, you display the form to add a child post directly in place in the template for parent posts.
You can't then use a URL parameter to pass the ID of the parent post.
But in the form, before its submitted, the "current" post is determined by the context of where the form is inserted. So if you insert the form in the template for parent posts, the parent will be the current post.
So, in your form for child posts, in the settings for the parent field selector you can use the wpv-post-id shortcode to set the default value for the parent post selector to the current, parent, post (second screenshot).
Either of these should work, depending on whether you want to link to a form to add child posts, or to include the form directly.