Sorry but there are no seedless blackberries or raspberries. The few that might be touted as seedless (ie, Rubus idaeus) still have seeds although they might be quite small compared to other varieties.

btw, blackberries and raspberries are typically woody stemmed, thorny perennials, commonly referred to as brambles. They are not trees!!

^^ This. Never heard of a seedless blackberry or raspberry - thornless yes, seedless no. The seeds of some varies are quite small, though -- "Caroline" raspberry comes to mind.

Zone 5B is marginal for many blackberry varieties. Raspberries are fine there.

Boysens are very plump juicy berries with relatively little seed content.

Thornless Logans also have less of a seedy sensation and are similar to raspberries.

I don't know if either would thrive long-term in 5B.

These are not seedless, as I'm also not aware of seedless varieties. But I picked up 3 of these plants last year at the garden center. I purchased a blackberry, raspberry & a blueberry. These are designed for planting in any garden. They are pretty & edible shrubs. I have a 6 year old who doesn't like a lot of fruit, but he loved going around looking for things to eat in the garden. It was great for him to learn about food growing; and I liked that they are attractive to grow in pots or mixed perennial beds. Here's the link - they were readily available at many places last year. https://www.bushelandberry.com/varieties

Have you considered blueberry bushes? They take a couple of years to start bearing but once established are easy to take care of. Though not technically seedless, the tiny seeds are so unobtrusive that they might as well be. Besides, blueberries are a favorite of most kids (and adults). Birds love them too, even more than raspberries, so you might have to protect them from the birds.

Seedless grapes are the only berry-like fruit I can think of that would be totally seedless. There are sour cherry bushes available on the market and it is easy enough to avoid the seeds of cherries.

Rodney

If you have acid soil, blueberries are a good suggestion.

If your family likes kiwis and doesn’t mind their tiny seeds, hardy kiwi are grape sized with smooth, not fuzzy skin. Kiwi plants are vines and need a sturdy structure to climb on.

Strawberries are low growing perennials which also have tiny seeds.