I found a frame for my granddaughter. It isn't a bmx, but anyway it has 24" wheels. (One is missing so I get to build a new wheel) I measured and the ISO is 520.
After I get the wheels done I'd like to find a nice, smooth tread tire. One also that will tolerate decent pressure. I've seen the Schwalbe but I don't want to go quite that skinny.
Thanks.

I'm not sure I understand your reply. The ISO is 520.
And, I did go to my local bike shop, where I shop and spend money on a regular basis. No real concrete answers.
And oh yeah, I'm self employed, so I can use the internet whenever I feel like it.
Thanks.

Kenda makes a couple of high pressure tires that would work for you I think. The Kranium and Kiniption are the models, I think. I'll see if I can find a link.

Kenda Tires | Bicycle | Find a Tire

according to sheldon brown, and his list of iso. There is no such thing as a 521. There is a 541. There is, however, a 520. FIVE TWO ZERO

Making sure that a bicycle tire will fit the rim

I get the feeling you are trying to say that a 520 doesn't exist. And, frankly, you're wrong. I had a Bacchetta Corsa 24" that used that size (by ISO measurements) for both wheels. Also, Terry bikes of earlier vintage that had 24" front wheels used the 520 size. The Terry website still sells the 520 tire, but it is a bit thinner than what I want.
My post was NOT to question the ISO of my wheel...my post is asking if anyone knows a source for a tire that will fit that rim size.
I don't know what your objective is, other than insisting on being RIGHT (you're not)...but. just. go. away...Thanks but no thanks for your "help".

All I find on several sites without knobby tread is the Schwalbe Durano 0.9 inch. Slim pickings for 520 tires.
Another choice in 520 x 1. https://www.sheldonbrown.com/harris/tires/520.html
Still slim pickings of slim tires.

Niagara has an ISO 520 tire by Panaracer in 24 X 1" for $18.80. Looks like Terry has the same tire. There may be a Specialized Nimbus for that rim, but all I find is a reference to the tire as 24" without details of what 24" size it is.

Looks like there are about as many incompatible 24" sizes as there are 26" sizes. What a mess.

Are the Terry Tellus tires (520 1 1/4") insufficiently smooth?

The terry Tellus so far seems to be the best available choice. I wasn't aware of the nimbus. Will check that out. Thanks.