Our almost-six-year-old is already in multiple marketing databases, and it’s all my fault: Once our daughter couldn’t depart with us for free, we started signing her up for frequent-travel programs. The price of miles and points are already baked into the tickets we buy for her, so we might as well take part–and besides, you’ll never hit million-miler status if you don’t start sometime.

But tending these accounts has been more work than I imagined, because some companies have a hard time grasping that children represent a special group of customers who can’t be expected to have their own e-mail addresses.

At first I thought I’d solved this problem with “sub-addressing”—creating a new e-mail address on my existing Gmail account by adding a plus sign and additional text to my username. It’s an Internet standard, and I had no issues creating accounts for our daughter at United Airlines, JetBlue, American Airlines, and Amtrak with a “plus-ed” address.

But when I tried logging into our daughter’s United and JetBlue accounts a week ago and was greeted with various errors, I saw that both airlines had stopped accepting sub-addressed e-mails.

The problem was worse at JetBlue, since your TrueBlue ID is your e-mail address. I had to call and provide our kid’s account number and the no-longer-accepted e-mail address; the rep told me she needed her own e-mail address but then accepted a version of my Gmail account with a dot in the middle of my username. It’s weird to have to go through such a workaround when JetBlue’s site has a separate workflow to create a child account.

At United, I could change her e-mail to a dotted version of my Gmail handle after logging in, since MileagePlus account numbers double as usernames. United’s Twitter account then told me I could have put in my own e-mail for her account from the start. I would not have guessed that, since UA’s account-opening UX assumes you’re a grownup–and the e-mails sent to our kid suggesting she jet off to the likes of Australia, Brazil and Israel don’t exactly speak to the under-10 demographic.

Meanwhile, Amtrak and American Airlines still seem to tolerate plus-ed e-mail addresses. (I can’t speak to Delta, as that airline’s network doesn’t work for us.) But after the last week, I won’t be surprised if our little one gets unexpectedly locked out of either account; I just hope I don’t have to spend too much time on the phone to fix that problem.