Some people just never give up the drama, do they?
Parenting orders have effect until the children turn 18, unless the orders are superseded by a parenting plan, amended by consent or changed by the court before the children come of age.
This means that legally, there's no age below 18 where it's acceptable for either parent to unilaterally stop the children from spending time with the other parent, and doing so without valid reason constitutes a contravention. "The child said he/she didn't want to" is not a valid reason.
However, what the other parent may be giving substantial weight to is the court's consideration given to children's views when deciding what's in the best interests of the children.
Basically, if a child is 12 years of age or above, they are in most circumstances deemed mature enough and informed enough to decide for themselves where they want to live and how often they want to see the other parent. However, if you have orders already, as in your case, this consideration by the court is not one to be decided unilaterally by the parents. The parents must continue facilitating the children's time with the other parent until those orders are changed.
If the children 'decide' not to spend time with you, and the parent decides to suspend their obligations under the orders to facilitate them spending time with you, you will have access to a pathway to have the matter resolved.
I always suggest mediation first, especially in this case. If the parent follows through on this intimidation to withhold the children's time with you, you can request a child-inclusive cpnference so that both parents and a third party can ensure the children's views are heard and respected in a supportive environment. If the parent is saying, "Dad doesn't want to see you this weekend", that will probably unfold as an untruth in a child-inclusive conference.
If the issue isn't resolved, you can file a contravention application. The penalty in the first instance is ordinarily a bond of good behaviour for a year, but if the orders continue to be contravened, it may result in a change of living arrangements.
Often, contraventions where it's the children saying they don't want to spend time with the other parent will result in a potential change to the orders. Don't be afraid of this as a possibility - the reason for the children making this decision will likely be thoroughly investigated by a family reporter, and if what you say is found to be true - that the mother has discouraged that relationship - it will look very damning for her.
However, if the children attain 12 years of age before any of this happens, then the court may give significant weight to their views if found to be genuine, which could result in an order to the effect of: "That the child/ren spend time with the father/mother in accordance with his/her/their wishes".
Try and remember what it was like being 12 years of age before you start to worry too much. Kids don't like to have their parents criticised, so if one parent is doing it constantly against the other, it is psychologically more likely to be their relationship that falls into disrepair, not yours. It's unfortunate, of course, but it's not something you can control. Always be the supportive parent who encourages and respects the other parent, no matter how challenging things get. Your kids will thank you for it when they're older.