After reading the report, there is no indication they were coached.
The CPS lady asked if they knew there were drugs in the home, they said no. Then asked something like if they themselves had ever done drugs, they said no. Finally they asked so if the test is positive it means you have been exposed to drugs by other people without you knowing, which they answered affirmatively. Standard questioning and standard answers.
If they respected the rule of staying out the master bedroom, which is sounds like they did, no matter how erratic or scary Nick and Kayla were acting, it is very realistic that they would not suspect drug use. If you're not in that environment growing up it’s just not something you will randomly suspect even if true. The oldest is 16, which is almost an adult, but kids are still naive and still learning at that age. Plus sounds like he was out of the house more than the rest. The younger kids would just accept anything. 100% they know Nick drinks a lot, but drinking is normalised in broader society in a way drugs are not.