Line 21 is where I report my UK pension and I didn't have the IRS come back to me. I enter the words UK pensions to describe it. If they do come back for more explanation, what else more could I say?
When I consulted IRS (not in an audit, but it could have led to that) I was told to assimilate all my state pensions from all countries subject to Totalisation Agreements to US Social Security, and include them all on line 20.
On various online forums various experts have disagreed. But over 10 years and more than one examination this has never been questioned by the IRS.
The issue is this: the IRS (Treasury actually) is charged with interpreting and applying the tax law. They are not always consistent and until and unless there is a Regulation, or with respect to a specific taxpayer a Revenue Ruling, it works. At least for some.
I am not saying this is right; just that more than one IRS officer has said it is. And let me add this principle of administrative law: the USG is not responsible for the errors of its agents.
I have been to Tax Court, and won, on other issues. I'm not sure there would be enough of a tax result for the IRS to join me in litigation on this one. Except that everything on line 20 is effectively taxable only in the UK.
For the rest: the issue is whether the benefits were paid to the parent in her own right or as trustee for the child. In the case of the (former) Child Trust Fund (£250 + £250) it's clear that it's taxable in the USA, if at all, to the child. It is not taxable to the parent as child's income unless the child is, himself/herself a taxable US person.
For the parental benefits it's obvious that if there is a non-US parent, it's that parent who should claim the benefits, if possible.
Finally there's this: from the standpoint of a tax collector, every bit of income should be taxable
somewhere even if the effective rate is zero. (That's indeed an OECD principle, it can be looked up.) It is when it is not taxable in any country with a "substantial" marginal rate of tax that the civil servants who run our tax agencies get exercised.
But perhaps I'm being too philosophic. It comes with age.