Funny how many Democrats (like you) are now in the "Do not trust the government" camp ... after it was the Republicans under Obama.
It's more for certain actions which he has displayed willingness to engage in. Why would I think undocumented immigrants should trust this? Trump made immigration a central pillar of his campaign. Of course they shouldn't trust him. If Obama had run a similar campaign, it would be equally as true
(actually, given the deportations under Obama, it should be true either way).
Trust in government has to be based on the people who run said govt to some degree.
The Census operation is now pretty safe and guarantees privacy for those who participate. Even Trump is bound to this. If he cheats and instructs Census officials to submit data of undocumented people to ICE, this would be against the law and a massive abuse of power, which would result in court cases and impeachment.
Court cases maybe, impeachment is doubtful unless Democrats control the House, which is still almost meaningless unless they can line up enough votes in the Senate. Also this depends on him winning reelection as well, which isn't really a given right now.
Given that Trump has ICE at his disposal already, and that the idea of more deportations from this census change depends on who is POTUS post-2020, my real concern is throwing in the census question for cynical and/or misguided reasons, which can reduce the response rates from people who don't trust the government. This can include families of undocumented immigrants - legal, citizen family members.
I'd be more OK with this if they added it now for the 2030 census
(not the one coming up in just ~2 years). That way they can do numerous rounds of fully-funded testing, and not cheap out on it. I'd like them to prove that it won't cause problems. I'd also have less of a problem with this if Congress passed a law barring citizenship data from being shared with states for redistricting purposes. I'm not taking their word for it, and I don't expect a lot of other non-Republicans to either.