While I cannot speak for how things work in Texas, I may have spent a few years as a child protective caseworker (NY). First, it an extremely rare occurrence we are surprised/taken aback by the allegations. That story, while a little childish, it's not that bad. I know most will advise otherwise, but sometimes (depending on the seriousness of allegations) the best thing to do is 'surface-cooperate.' Let the caseworker in, be fake-friendly if necessary, present any evidence that shows you are a decent family--sometimes that may be giving them the name and phone number of a family friend or uncle (obviously someone safe and reliable for this) or even an older sibling (who might now be an adult living on their own) who can say that wasn't a typical occurrence.
A large portion of the job is making decisions on gut feelings--how we present what is seen at a home to our supervisors can have a difference in the outcome. No caseworker wants to take your child. NONE. They are overwhelmed with what they have already. Removing a child means hours (if not entire days, or weeks) in court, not to mention (drafting a petition to court, filling out redundant forms for services, writing service plans, etc) in addition to the rest of their responsibilities
I have had families who are extremely uncooperative--which is their right. That said, I had to look elsewhere for information and ended up obtaining records that told me about a completely different situation that was never reported. One that there was now proof in front of me, so they ended up with an indicated report (not removal!). If the mom would have let me in, let me see the kids at home--I wouldn't have bothered to search for this and no one would have been any wiser to it.
If you appear to be mostly cooperative, I will do my best to respect any conditions a parent might as of me. Maybe you don't want your child interviewed at school--letting the caseworker talk with them in the next room is a reasonable option. I cannot speak for Texas, but in NY caseworker's do not need your permission to see kids in school and actually have legal protections in state law for cps and schools to ensure cooperation. While medical, mental health and chemical dependency information is mostly protected (unless they call in the report)--you would be amazed at the amount of information the caseworker can collect on people if they really try.
Again, not nearly enough caseworkers. If you have three times the caseload your state says you can effectively handle, you do not have time to do this digging unless you really need too. Being surface-cooperative can easily avoid this.
Lawyers are generally recommended if the process goes to court--unless of course, you have thousands of dollars you won't miss. Most people don't. If I was your caseworker, the worst I see out of this situation (if an accurate description) would be one or both parents having the report indicated against them. You say the child is not your son by law--if by that you mean you are not the bio parent. Meaningless in NY. A person legally responsible is someone who is 18 years or older who lives in the home who has (or had) some (no matter how minuscule) caretaking role. Maybe the 18-year-old sibling made dinner on occasion--that meets the definition. It doesn't take much--- i.e. pick the child up from school, bring them to the doctor's once, etc. But again, this can be based on caseworkers discretion.
Acting like you are cooperative, but politely refusing whatever might make you look bad is the best possible option in my opinion. Those who are very combative and make it a struggle for me, make us look places we necessarily wouldn't have.
So even if the caseworker indicates or substantiates the allegations (some credible evidence the maltreatment occurred), there is still what is called in NY, a fair hearing. It takes a higher standard of evidence to keep the report as indicated at a fair hearing--a preponderance of the evidence (50.1%). Many end up overturned for this reason alone. Lawyer would be beneficial, if financially feasible. If it's a borderline case if I debate whether or not it should be indicated, if I know the parents would more likely ask for a fair hearing, I will bring that up in a discussion with my supervisor about it. Cooperative families who I am skeptical about it passing a fair hearing--more likely end up with unfounded reports.
The process isn't fair, but it's how it works. It takes a lot of remove a child and most parents have had numerous chances to change but refuse. Explaining why the situation had little to no effect on the child is the best way to handle the situation. The question my supervisor asks me if we are deciding the indicate---what is the effect on the child? No effect, unfounded report. Sometimes one-time occurrences that can obviously be a substantiated report--we chocse to unfound.