My understanding of a U.S. Sanctuary City is that crime and criminals are not tolerated but individuals won't be sought out specifically because of their immigration status. They believe that's the Federal Government's job
Sanctuary cities aren't exactly welcoming illegal immigrants with open arms. The hope is that existing illegals will not be afraid to report crimes that they are aware of or have been victims of, use health facilities and enroll their children in schools, for fear of being deported.
Studies have shown that in sanctuary cities, there is still a general distrust of the government by illegals and the sanctuary status of the city they are in, has little effect on improving it's crime rate or it's economic situation.
I'm not sure why a local government wouldn't want to assist the federal government with enforcement of immigration laws of the country they are a part of and claim to support.
I don't imagine there is anywhere in the world where I could just sneak in with my family and somehow expect to put my kids in the local school and take advantage of the medical services etc and not be asked/told to leave ( I'd move to Tahiti if I could do that, LOL! )I'd expect that the lot of us would be apprehended and eventually deported. That would be part of the risk I took. Yet when people do this, rather than blame the parents for doing something highly illegal, there are those who criticize our government for enforcing it's laws and the problems that arise when the existing system becomes overwhelmed.