Many of the sites want you to purchase materials or sign up but also have many free materials that you can use to help your child familiarise themselves with the test. Please bear in mind that different local authorities have different tests so you need to look at the format of the Kent questions. Look here for the Kent sample materials:
ELEVEN PLUS EXAMS
A site with a wide range of advice, tips and links to other sites:
This site also has a guide to say which type of questions are included in the Kent test which can help when you are using free or purchase materials.
Try the free demo for free non-verbal reasoning questions: https://www.elevenplusexams.co.uk/free-11-plus-practice-papers/elevenplusexams-non-verbal-reasoning-nvr
BOND ASSESSMENT PAPERS
You will need to register to access the free resources.
MASTER BRAIN ACADEMY
Non-verbal reasoning paper with answers