Does the Roman Catholic church consider HIPAA to be an unjust law?

There's no clear-cut, doctrinal, or official Roman Catholic stance on HIPAA. In general, Catholic organizations willingly comply with HIPAA. To that point, a quick search shows a lot of Catholic organizations describing their members' rights under HIPAA.

More notably, if there were a doctrinal issue with a national policy like HIPAA on any grounds, that of subsidiarity or otherwise, I would expect to see a statement and/or resistance from the United States Conference of Catholic Bishops. For instance, there are numerous statements and commentaries opposing portions of the Affordable Care Act. Not so with HIPAA. The only mention on their site is a wellness document, in which the rationale for requiring "new sisters" to provide HIPAA and DPOA forms is summarized like so:

We bind ourselves to the members of the community, trusting in their faithfulness to support us.

So, it seems to me that the USCCB, or whatever subsidiary this is, feels no need to object to these policies. And to the contrary is highly respectful of them, pointing out that the HIPAA release papers demand is justified by the intended level trust and support in their community, which implicitly makes the request a bit extraordinary. Whether that level of respect for HIPAA and medical privacy extends to every "application" may be a different matter.

So, there are two things to keep in mind:

1. Subsidiary is a single, vague social guideline among many; not an overarching, specific moral precept. Subsidiarity demands that we handle things as close to the issue as possible. But, the motivation to escalate to higher levels of community and government stem from the Catholic principle of solidarity. And both of these principles are meaningless without the context of another moral need or needs: In this case, the right to individual privacy, the responsibilities of a parent, and the obligations of medical practitioners.
2. The Church explicitly avoids speaking on most matters. You're expected to reason things out and use your conscience. There's a lot to think about. The specific issues you're familiar with may or may not be representative of the broader application. Without overanalyzing, acknowledge the complexity. And vocalize that our judgement if it seems imperative.

There may be moral issues with certain applications of HIPAA. Presently, the moral issues are either isolated or negligible enough in the eyes of the Catholic Church to warrant widespread Catholic compliance.

As to whether HIPAA erodes parental rights, maybe. Maybe not. With respect to parental rights and responsibilities, I would personally be inclined to argue in favor of HIPAA or HIPAA-like legislation for two reasons:

1. HIPAA allows kids to receive medical care without fear of disproportionate punishment and judgement. It's not all that dissimilar, in my opinion, from the seal of confession. Like every parent, I'm far from perfect. And we tend overreact and punish for more than an offense is worth. I don't want the fear parental overreaction to stand in the way of medical treatment. (Or spiritual treatment!)
2. My kids will know the Catholic stuff either way. They will make mistakes. Some of them may be downright egregious. But, they'll know my beliefs regardless of whether I'm informed enough to distribute a consequence for every mistake. And because I'm far from a perfect parent, far from an effective disciplinarian in emotional situations, it's probably better in some cases that I don't know what they've done, and rather allow the kids to reflect on their own actions based on what they know I/we believe.

And to the extent that any parent succeeds in charitably correcting their kids' behavior with due mercy, I presume the kids will be open about their lives. To the extent that any parent reacts selfishly, degrading, devaluing, or disproportionately punishing, I suspect that kids will be private about their sins.

HIPAA lets doctors do their work without forcing them to double as spies for parents. And it lets kids seek needed treatment without worrying that their doctor is a spy! And this agreement, I suspect, does not stand in the way of a healthy parent-child relationship.