It is a myth that children can decide how often they see a parent. There is no magical age when they have the right to decide who they live with and with what frequency they see the non-custodial parent. If your daughter has valid reasons for not wanting to see her mother every other weekend, the Court will take those reasons into consideration when determining a partial custody schedule. Sometimes, with an older teenager, the Court will decide that the teen should spend a certain number of waking hours with the other parent per week or per month, thereby modifying the typical every other weekend routine to make it easier for the teen to work, spend time with friends and become involved in activities. Rarely, if ever, is such a schedule entered for younger children. In short, if your child simply does not want to go, you must encourage her relationship with her mother anyway. Above all, do not break the Custody Order. Perhaps you should speak to an attorney in your area, and determine if the reasons your daughter doesn't want to go to her mother's would be considered compelling enough to modify the Order. An up hill battle, I am afraid. Even if the order is modified in reference to your daughter, absent extremely strong reasons, it is doubtful that the 5 year olds time with his mother would be changed..
There is no magic age when children get to decide custody. The best interest of the child is the standard. The judge will decide.
The law regarding a child's 'right'* to choose is a matter for each State and jurisdiction. The judge in most States, not the child, makes the decision based on the best interest of the child. Although not a standard by any means, many States have begun to give 'consideration' to a child's declaration of custodial preference when the child reaches the age of twelve or thirteen, sometimes fourteen. There are even cases when children of age 9 are allowed to testify.
The judge is normally given almost unlimited latitude in whether or not she or he listens to a child and how much weight to give to the child's wishes. In short, there is no specific "age" but the younger the child the less likely for a judge to give the stated preference much weight.
Good luck to you.
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