When parents go through a divorce, the worse thing is to involve the children, or God forbid, poison them against any side of the parents or their family.
Children should never be dragged into such problems, and must be kept neutral. They have an obligation towards their parents, and must honour and respect them both.
It is equally healthy for the children to have access to both parents, and this will be something very positive for their upbringing.
If things are bitter between you and your ex-husband, suppress your anger and show them the better path by pushing them towards their father as well.
If you in any way prevent your children from having a relationship with their father, then you are committing a sin and doing wrong to them more than anyone else. If your children themselves do not want to interact or contact their father, then they will be sinning, and you must encourage them to maintain a relationship with their father. They must stay as committed and loyal to both parents as much as possible, and this is something wajib.
This is of course in normal cases where the children are safe, and there is no justifiable reason for them to stay away from their parent. In the case of there being a legitimate shar'i reason for them to be distant from their parent, the situation would be different and it should be dealt with in a different manner.