He was pleading with the judge for leniency and to get his kids back. Citing that the kids said they were unaware of drug use totally makes sense in that case - it shows that Nick and Kayla were hiding it well and being "responsible" versus snorting rails right in front of the kids.
However, "And uhhhh the kids said if they test positive it must have been from something else, not their parents!" made NO sense to tack on to the end there given the context. And don't let him tell you he was "quoting" the state - he wasn't, he was paraphrasing from memory. He intentionally and deliberately included the "if they test positive" line because he knew there was a kid pissing hot right around the corner and he was trying to get out ahead of it (as he is wont to do - see confessing to choking his son in the chiropractor's office lobby as a more recent example)