I gotta go with Blondie Bear on this one. Of course, that's a usually usual thing because she's smart.
Social workers are here to try and spot what could be a problem to make sure children aren't being raised by the wrong people. Surely, you'd have to agree that plenty of people DON'T deserve to raise children or that there are homes a child shouldn't be in.
If we look at the scenerio from an outside view, there are issues.
Dawn's absenteeism is a red flag for a lot of things. Think about it. A child being raised by a "capable" person can USUALLY insure they actually show up to school. When they find out said kid is NOT going to school, they would take steps to remedy that.
Buffy is all scatterbrained when she shows up. Sure, that happens to the best of us. On the other hand, it can also be a red flag for other things. Substance abuse for one. Probably ALONE the scatterbraininess wouldn't have been so bad.
Then you have Spike. Who's obviously acting EXTREMELY weird and, although I love him, NOT the man I'd want influencing MY teenage daughter...just on site. The fact that he was acting like a mad man switching from one personality to another and carrying a "blankey"...not too coool.
Buffy should NOT have lied about Willow. She should have just said one of her friends lived there to help her out. By lying, she looked like she was hiding something.
In the end, you have THREE grown adults in a house all lying and acting totally bizarre. The normal thought? They're hiding something. And they WERE hiding something. Since Doris doesn't know about Vampires, witches and Slayers, she had to assume they were hiding something normal. Like, perhaps, "magic weed".
Under NORMAL non-mystical situations, this would all be a HUGE red flag to anyone. This would NOT be a scenerio where I would walk away assuming I misread everything if I were the CPS woman.
To be totally honest, if I were her and DID know and believe about mystical things going on, but not where Dawn came from, I'm not sure I would think it's a good place for Dawn. We have a Slayer who's not always able to be around, a witch going crazy and a vampire who's in the process of going less crazy and danger surrounding all three of them. Dawn WAS getting lost in the shuffle. She's not the picture of stability even though she got to stay. Strickly from a parenting stand point, it's not the best environment for her...and her father probably SHOULD have been tracked down and forced to take Dawn. (He SHOULD have been sending Child Support if nothing else.)