Duplicate entry '0' for key 2
you are correct the error you are seeing is due to the fact that you are not inserting the practise_id into the table reviews, to solve this issue add a hidden field to your current form like so:
<input type="hidden" id="practiseID" name="practiseID" value="<?php echo $_POST['child'];" />
and then use this form in your database insert.
There is potentially another issue though as you have a field reviewers_id which will never get a value the way you have things set up currently as the standard dreamweaver database insert doesn't have an inbuilt method of creating a variable (session or global) that you can use to get the reviewers id from the insert into the reviewers table. It is possible to hand code a solution to this but I suspect that would not be your preferred option so if you have data assist you can get around this very easily by using the data assist insert server behaviour to do the insert to the reviewers table first with a blank redirect, this creates a session variable for the id of the record created that you can then use in the 2nd insert to the reviews table.
Hope that makes sense.