Young children at this age are starting to become more independent and will have a basic understanding of relationships between different people. Their language skills will be better developed and they will have a basic grasp of time. They won't, however, be able to understand the concept of divorce or separation.
One way to help your child to understand the parenting time pattern is to make a time chart with pictures to show what they will be doing on what day. A picture to represent mum and a picture to represent dad will allow them to see the routines that you have set.
Children at this age are much more comfortable with spending longer periods of time away from their main carer. This means that two or three consecutive nights a week with the other parent are usually fine.
Try to help them to understand the parenting patterns through language and pictures. A pre-bedtime phone call to the other parent might help your child to adjust.