here is the law as to how the child support is ordered payable. it is retroactive to the day of filing of the divorce. that means if she files today, a court can order the child support payable back to that date of filing of a divorce. so to answer your question - yes. she can get it retroactive with arrearage child support -
however, it may take into consideration any money you have paid "voluntarily" but it does not have to. you will need to be able to substantiate every payment and why - and, if those payments are par with what the guidelines recommend, then a court will apply those toward any arrears or back child support. but the court does not have to. they can consider it a gift. it all depends on how it is presented, how much you have paid to the finances of the child and if it is in accordance with the child support guidelines
25-320. Child support; factors; methods of payment; additional enforcement provisions; definitions
A. In a proceeding for dissolution of marriage, legal separation, maintenance or child support, the court may order either or both parents owing a duty of support to a child, born to or adopted by the parents, to pay an amount reasonable and necessary for support of the child, without regard to marital misconduct.
B. If child support has not been ordered by a child support order and if the court deems child support appropriate, the court shall direct, using a retroactive application of the child support guidelines to the date of filing a dissolution of marriage, legal separation, maintenance or child support proceeding, the amount that the parents shall pay for the past support of the child and the manner in which payment shall be paid, taking into account any amount of temporary or voluntary support that has been paid. Retroactive child support is enforceable in any manner provided by law.