From what I know, having PTSD disability and receiving SSDI does NOT, automatically, disqualify you for custody of your child. They are two different issues. And you are savvy to know that the agencies may be in communication behind the scene, IF there is any concern. I think everything is evaluated on an individual basis.
To reduce your fears, in addition to having and asking a lawyer, it may be useful to speak to an independent social worker (best option) or a social worker at Child Protective Services.
During your SSDI application you can point out, and answer directly, the ways you appropiately care for your child. The independant social worker or lawyer can help you prepare for SSDI questions on this matter. And you/ the social worker/lawyer, can call or look up, on the Child Protective Services (CPS) website, to assist your concerns.
Meanwhile, if you can, support self-kindness; let worrisome thoughts signal you to breathe, and remind yourself of the good actions that you take, for your child, everyday (respond to their emotional and intellectual needs, prepare meals, take them to doctors, to school), despite your diagnosis.
P.S. if helpful, there is a company called Alsup or Alsep (google for web address and proer spelling) that does all the work for submitting SSDI claims, to prevent the usual rejection with first application. They do take a percentage of your eventual checks, but friends have said it is worth it. Your lawyer may offer the same service.