Thank you for the quick response.
For other reasons, I restarted the AirPort Express (and I assure you that I had tried that multiple times before), and now both iPhones have connected. Also, I had already discovered the Private Address setting, been bewildered, and turned it off to no avail. However, both iPhones now have it turned on and are connected. (I have just installed iOS 14.4 on the older iPhone, so that could be why Private Address had been reset on it. I have downloaded but not installed iOS 14.4 on the newer iPhone, and I don’t know why it would have been reset.)
For what it’s worth, the new MacBook Air, which had connected previously, would connect but have a self-assigned IP address today. That was the reason for the AirPort restart, which seems to have fixed all problems. As I said, I had restarted it multiple times before posting my question. Gremlins!
Edited to clarify. When I posted the above comment, Timed Access Control was turned off. (I needed to turn it off so the iPhones could connect to Wi-Fi and download iOS 14.4, then forgot to turn it back on.) I turned Timed Access Control back; the iPad reconnected but neither iPhone did until I turned Private Address off. (The iPad already had it turned off.) In summary, it seems like @ddmiller was spot on with the suggestion to turn off Private Address. Why didn’t it work when I did that earlier? I don’t know, but perhaps enough restarts of devices plus putting iOS 14.4 on the iPhones and turning off Private Address all worked together to fix things.