There is no single answer. There is no legal requirement to. It is your business. But if/when she finds out you are likely to be asked why you kept it secret.  The practical issue is whether your children come to know about this relationship. In most cases it is sensible to keep your relationship with a new partner separate from your relationship with your children for quite a while until you know what the new relationship is going to be.  Here are a few more ideas.

For older children, it is sometimes best to pick up and drop off children at school, rather than at the house of the other parent.  Handovers can be a hard to manage both for parents and children, and if arguments cannot be avoided then this is one way to make things easier.  Children can find the transition from one parent to the other very difficult, as it a time when their loyalty to each of their parents is tested.  It’s not unusual for children to become withdrawn or distressed at this time, which sometimes leads to both parents getting the false impression that something is wrong in the other household.  Try your best to remain calm and positive if the handover is with the other parent, and if difficult things are said just don’t be provoked into responding.  Attending a Parenting Apart course or training organised by Shared Parenting Scotland helps to build up the skills to deal with these situations.

Look on your local council web site to see if they run any suitable classes, or try contacting organisations like Parent Network Scotland or Mellow Parenting. A Google search should show up smaller local organisations in your area. You may find that some classes say that they are only for parents who have contact with their child – try politely and firmly to persuade them that you need the training to help resume contact.