If I were the ACLU, I can't advocate going to trial or not going to trial without the benefit of reading the Complaints, Answers, depositions, expert reports, etc. What I will say is neither party has an idea of the likelihood of winning/losing. I've worked on over 15 class action lawsuits and at no point could I (or anyone I've ever worked with) put a probability on winning. Each side has strong and weak arguments and strong and weak case law to support their arguments. When a 3rd party is involved, trying to determine how they will rule is folly.
But in this case, if I were the US, I would not accept any settlement offers because I want a Court ruling on whether or not the United States can separate families who illegally cross the border. I also want to take my chances that I can win the case to deter other families from trying to border cross for the sole purpose of suing the US to try and get "free" money.