It's not "abrupt." It's precise to what you stated. It's to address this via a text-based forum, because we are not face-to-face.

I pointed out what you stated in perspective to what is provided for. You need to do that math to see there are Errors to consider.

"When the stimulus was issued there were 4 qualifying dependents"

I don't know what this means. You told us "a client" and "2 dependents." Now there were 4? And your client = 5 people? See how this gets confusing, fast?

The payout was in advance of Reality. The 2021 tax form(s) are the Actuals, and the reality. Use the payouts in filing: "here's what we are entitled to, and here is what we got."

It really is not Harder than this.

"I'm looking for the best solution to not negatively impact the parents or the aged-off child."

No. You are going to use the Right Filing status for what applies, for each person whose tax return you are preparing.

"So I am looking for a way to report this so that both the child and the parents aren't penalized for this."

What makes you think anyone will be penalized at all? Are you confusing the Child Tax Credit advance with the Stimulus/Recovery/EIP advance payments? Because CTC has a repayment provision. There is no repayment provision for EIP, so far.

Until you do the tax forms, you won't know what is going to happen. You don't get to pick from options. You file what applies.