I found this source as a good primer
Lots of room for interpretation by a social worker and police officer.
Generally though it appears they have no right to enter your property without a warrant if there is no apparent threat of immediate danger to the child is evident. Supposedly an anonymous complaint is not sufficient evidence to believe that the child or children are in imminent peril.
On the other hand if the visit involves something administrative such as whether your house is up to some kind of safety code than you can not refuse them entry.
Seizing of your child is even more open to interpretation.