Clearing up a few things after extensive experimentation this AM...
1) EA is locking XBox child accounts out of Battlefront 2. Their referral to the XBox parental account management process is disingenuous at best, because Battlefront 2 is ignoring the parental permissions provided by Microsoft. They are simply looking at the child/parent indicator on the Microsoft account and if the account belongs to a child (regardless of permissions or ratings settings), it is locking the account out of Battlefront 2’s online content.
A proposed fix elsewhere in this thread - linking the child account to a parent account in Origin - is no longer operative. That option has been removed from the Origin Mac, PC, and online clients. EA now handles account linking internally with not-very-solid internal routing rules.
2) EA is not all that serious about COPPA, because if you set up a separate adult Microsoft account and link it on through to XBox and EA, EA merrily provides online access to that account, even if it has the same contact information and names as the previous child account. Setting things up this way is non-trivial as Microsoft, to its credit, does check on this, and does not allow editing of birthdates for an existing account. But it is possible for a determined site administrator.
3) As to why EA might be taking this stance solely on Battlefront 2, well, that’s apparent if you think about it. Battlefront spent a whole lot of time on micro transaction economics (and quite obviously no time on account administration) in an attempt to boost their margins and stock price. This was controversial of course, to the point that Congress is threatening legislative remedies that will no doubt be onerous. And such Congressional actions will expose EA to liability if loot box gambling is found to have been made available to minors. So the simplest thing to do is just lock all minors out and wait out the storm.
EA stock has been getting hammered in the markets, and based on this experience, it’s entirely justified. I’ve worked around their half-@#$ restrictions for Battlefront 2, at the cost of significant parental control. But I’ll be damned if I spend my family’s money on another EA title. Considering Battlefront 2’s market is young emerging gamers, who stand to spend a lot of future gaming dollars, that’s a dumb approach on their part. But it’s no longer my problem.