From your screenshot I see that you want to have your Parent taxonomy terms and Child taxonomy terms to be separate filters.
Unfortunately this won't be possible to do without some form of custom code. The main reason is that the Parent and Child terms are a part of the same Taxonomy so they cannot be separated into 2 separate fields.
The only way I see this possible is by manually creating the Taxonomy fields on the form with the same Name/Slugs of the terms on the backend and using custom code to manually populate the taxonomy term of the created posts using our Toolset Forms hook to pickup the field data.
Given that it would involve the use of custom code, I won't be able to assist with this since it would be out our support scope.
I have about 25 parent terms with children term assigned to them. This means that the dropdown select will be very long and not so user friendly. Is it possible to accomplish something simular to the image in the same field, like levels
Yes it is possible to achieve this, however it will involve the use of custom code.
What you will essentially need to do is to split the taxonomies into different dropdowns on your form using the Generic Form Fields option, populating one field with all the Parents. This would mean populating all 25 Parent terms into a Generic field with their Names as the display and the ID as their values.
Then you will need to do the same for each child term and wrap those child term fields into a conditional statement.
When you do this then the child fields will be dependent on the Parent dropdown selection rather than having all the fields display at once.
