Your ex having played a large role in Isabella's life will have a right to spend time with the child, or I should say she will have a right to spend time with him.
In these matters it is not the rights of parents, because parents do not have rights. It is the right of the children spend time with their parents.
The mediation is the correct way to go to try and resolve the issues and you are going to have to sit down and discuss what would be the best for the children during the mediation.
Your 13-year-old may be a problem, because if this matter goes to court her wishes will be taken into consideration when the court makes its decision.
It is a matter where any arrangements will need to take into consideration the shifts and they will have to provide you with the rosters when they 1st come out and any time the children spend with them will then be arranged.
You have to understand the law, when it comes to children's matters, is interested in what is in the best interests of the child and the primary interest is that they spend time with both parents.
I cannot see with the shifts that they are working how equal parenting will work it is not possible.
If they are to be with their father, and not see him because he's working and spend the time with their stepmother this is highly inappropriate and the court would not agree to act.
It is a matter where if you can reach an agreement with the children spend quality time with their father then this is better than spending time when he is at work and being with his current partner.
The move to Newcastle may be an issue but it depends how far they will be from their father on whether the court will think the move is not in their best interests.
Is there anything else I can assist you with?