If you find that you can easily win every game against your child, then I would strongly recommend doing the following. First, DO NOT play easy and make intentional mistakes – that frequently results in frustration in many situations. Instead, change the starting position and make it a challenge that will help your child learn and still be something that they can ‘win’ at.
I would recommend starting the game with your child having all his/her pieces, and you having ONLY your king. Have your child count the number of moves it takes to checkmate your king, and write down the first time how many moves it takes for your child to do so. Then set a goal of how efficiently your child can checkmate you (should be able to do it in 10 or fewer moves).
Once that becomes simple, then simply add all the pawns to your lone king – and once again track how many moves it takes for your child to win – and so on. Adding both the rooks as well as the pawns – then both knights instead of rooks perhaps, etc, etc. This can be a fun challenge as your child will legitimately win without one side intentionally making blunders, and slowly increase your child’s ability to plan and find the best moves.”