Abba:
You wrote: “If it’s the children’s money and she can’t use it on herself then the father should not complain.” Correct. The legislation and the courts are banking on an assumption. It is expected that the expenses of the mother (can include rent, clothing, food, etc.) are enough that the money provided will be used for trhese expenses. If it can be verified that mother used the money for vacations and such luxuries for herself, the fatgher may have grounds to make a case. This is technically possible. But highly unlikely.