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I am sorry to hear about this situation. DFPS (CPS) cannot obtain a warrant. They are not a police agency. Just like before, you can refuse a drug test.
Understand that DFPS have no "power" per se. They cannot force an interview, and they certainly cannot simply take your child away from you without a a court order. However, they have a mandate from the state to stand in front of the Court for any child that they feel is being abused/neglected.
This means that if they feel that the child is neglected/in danger, DFPS may decide to take the matter to court and ask to have the court agree that your parental rights should be suspended (or even terminated in serious scenarios) until you show to the court that you are an able parent. In these cases, they can ask the Court to order you to take a drug test, yes. But that is only if they end up going to court.
Now, while DFPS has the "ear of the court," this does not mean that the court always agrees with DFPS and if you challenge their claim, it may be denied. But this is a risk.
If you feel that the requirement by DFPS is unreasonable, then you can refuse. If they then go to Court (as explained above), then you'd have to show that your decision is in line with "best interests of the child."
DFPS often uses misleading, cajoling, and threatening tactics, so be careful. An attorney is recommended if one decides to meet with them, because the attorney can quickly cut their attempts to intimidate the parent down. Be careful and do not let them intimidate you!
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