Is a 2020 i5 MacBook Air with 256GB worth it for my daughter?

The answer is going to depend quite a bit on what it will cost and what your daughter will be doing with it...

It will be able to run Sequoia and be supported for several years. It's a perfectly capable machine for web browsing, email, basic office apps, etc. With sufficient cloud storage for photos and such, it would be fine for someone in middle school or even high school. She's probably not going to need to run an y particularly demanding applications on it.

That Mac will not be able to run the next version of macOS - macOS 26 (“Tahoe”), due out this fall. Since it does not have an Apple Silicon processor, it won’t ever get Aople Intelligence (if you care about that). Also, these days, a 256 GB SSD is not very big unless one’s needs are very basic.

Offered for free? Sure. It will run Sequoia and Apple will provide security updates for a while to come. But I wouldn’t buy one, personally.

If you buy any used Mac, make sure the seller has done this:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support