just my .02, but i seriously think that she is just being vindictive. what grounds do they have for requesting drug tests? in our area, cps has to have cause before they will ask for them. is it possible that something on the home visit triggered this? i would submit, without hesitation, but i don't use so it isn't an issue for me. i'd be p.o.'d at the intrusion, but i'd protect my kids at any cost, and the children being taken away is a very real scenario.
we have actually been in this situation in the past, but had the case closed for "no cause." like you, it was a vindictive ex that made the call.