There are two separate issues here, the benefits of the intervention and the risks.
You are addressing the benefits. It’s true that we don’t know the benefits yet, but that’s minor. It’s hard to test embryos, they are relatively fragile. As the girls grow up, they will doubtlessly undergo lots more testing to determine whether or not the intervention was effective, i.e. whether they actually acquired some degree of resistance to HIV. Even if not every cell is edited, they may have some partial resistance. And even if there is no benefit, that’s often the outcome of clinical trials. Participants in clinical trials are often reminded that they may not personally benefit from the trial. It is the future patients that will benefit.
The greater concern is the risks of the intervention. This is what has other scientists up in arms. It’s acceptable if patients are not better off after a clinical trial, but it’s unacceptable if they are worse off. Unlike most clinical trials, here the risks are very hard to adequately assess until it’s too late. And the risks may well affect the descendants of the two girls.
The kids were born late October or early November. That’s not hard to arrange at all, just wait until January to start the project.