The thing is, if the court does appoint an attorney, the attorney will need to explain these same concepts to the child as part of preparing a defense.
There are lightears of space between doing your best to see that someone who can't defend themselves in court has qualified representation (even if there are difficulties that the representative will face in conducting defense), and pitting the same person against an expert litigation team without a qualified advocate.
The former situation is a good faith effort to give the defendant a fair trial. The latter situation is a sham trial.
The obvious danger, in a criminal hearing, would be for the accused to say something that they weren't required to say, and that hurt their case. That is one obvious reason people have a right to an attorney. I don't know enough about immigration law to say whether that's a possibility. In a criminal case, you have a right to remain silent. You don't have that right in a civil case (you can punished for contempt, and the judge is allowed to assume the reason you want to remain silent is that the truth would hurt your case ( https://en.m.wikipedia.org/wiki/Fifth_Amendment_to_the_Unite... )). I have no idea if you have the right to remain silent in an immigration hearing, but it wouldn't surprise me if the right is severely limited. If you have a right to remain silent in an immigration hearing, then it would be impossible to guarantee a fair hearing without providing an attorney.
To be honest, there are several things in the legal system that I don't think should be resolved through an adversarial process. Immigration hearings are a good example.
I would say that these unbalanced trials are pretending toward fairness, fairness theater. If your primary concern is the cost of these proceedings, then don't argue for taking away only one of the costly, yet crucial, aspects of the trial [taxpayers already pay for the judges and the state's attorneys (both of whom are likely making much more than state appointed defense attorneys would be), so refusing to pay for defendant's attorneys doesn't save a heck of a lot of cost]. Rather get rid of the unfair trial altogether and make the thing an administrative proceeding. If you think a fair trial is justified, then you've got to provide qualified representation for those who can't come up with it themselves. That's just the minimum bar for fairness in an adversarial trial. Meet it, or get rid of the trial altogether.
I don't have enough information to really have a strong opinion on whether these situations should be handled administratively or whether the state should provide attorneys. However, it does seem that the current solution is on the order of the most expensive possible solutions without managing to meet a minimum bar for fairness. It's a worst of both worlds compromise.