Things to think about and try:
Having their own wee space to relax in. It doesn’t have to be their own room, it can a den made from putting a sheet over a table and adding cushions.
Doing things together to have fun and relax
Having a predictable routine
Having scheduled fun family activities such as “Friday Nights at the Movies”/duvet nights at the telly
Eating together
Involving your child in planning meals and activities