Making a story book can be good at this age. Make up a little booklet with folded/stapled paper. Include photos or drawings (bad drawings are fine, just include some recognisable feature e.g. blue door, red hair, favourite toy) of the old house and your family. You always want to start the story with "This is X. X lives with his mummy, daddy, little brother etc" and a photo or drawing of all of you.
Then you do some pictures of the old house. "They lived in a house in X. It had a garden, 2 bedrooms, child's bedroom was blue, it was near the park etc"
One day, the family decided that they needed a new house. They looked at lots of different houses, but none were quite right (include some pictures of silly houses, maybe with reasons - too old, too skinny, too scary, too smelly).
Eventually, they found their perfect new house. It has a bigger garden, more bedrooms, closer to grandparents (whatever positive features about the new house). Child was able to choose the colour for his new room.
Sometimes child felt sad. He missed his old house. He missed the street and his friends and the shop. When he feels sad, he can talk to mum/dad and look at pictures of the old house. It can take time to get used to a new house. Sometimes a new house can feel strange.
But soon, the new house will start to feel like home too. We'll have Christmases and birthday parties and special teas and lots of games and fun together. Include picture of whole family smiling with new house.