You need to subpoena the phone company, which isn't easy and which takes quite a bit of time and precise compliance with the CCP.
If you haven't got a lawyer, it's time to get one, at least for this purpose and maybe for other purposes for your child custody case. Family lawyers sometimes offer "unbundled" services, meanign you can hire them just for specific tasks.
Since you want messages sent to your phone, you may be able to get documents regarding the messages without a subpoena, by providing the company with a release. You should contact a family law attorney for more specific advice about how to obtain relevant evidence.