You need to present the evidence that she accessed that account for child support purposes, and that it was your agreement that she do so. The evidence is in your bank statements. How you accomplish this depends on what court and court process you are in -- divorce court on a motion, child support court on a new case or motion, or something else? You need to take whatever you received wherein she is asking for the arrears, along with your bank account statements for that entire time frame, to an attorney in your area to determine how to best proceed. An attorney may be able to work out something that avoids court, or even bring a claim against your ex for false statements (for failing to identify the funds taken from your account).
Responses are for general information purposes only, and are based on the extremely limited facts given. A consultation with an attorney experienced in the area of law(s) indicated in the question is highly recommended. Information and advice given here should not be relied upon for any final action or decision, as the information is limited by its nature to the question asked and the fact(s) presented in that question. THIS RESPONSE DOES NOT CREATE AN ATTORNEY/CLIENT RELATIONSHIP, particularly considering that the names of the parties are unknown.