+2 for a Harris.
I've owned my most current Harris Bi-pod for over a decade and IIRC it's adjustable from 6" to 9". I highly recommend getting an adjustable model (allows shooting from most any position/terrain which is nice to have on a Non-restricted rifle).
My Harris bi-pods have always worked well and have mounted up w/o issue on my Ruger 10/22's + SR22 as well as my Just Right Carbine in 9mm and on AR's too. It's my go to unit for a rest, when not shooting from my bags.
They can add a little more weight then children may enjoy holding in a standing unsupported position, but are not to heavy for adults/older teenagers to shoot unsupported (at least for a little while) .
Also they have at least two models of mounting (again IIRC), one mounts to a basic sling mount post (more affordable and lighter version) and a second model which has the ability to mount to a picatinny rail or sling post with only a quick adjustment to the bi-pod.
I think they have at least two adjustable pole lengths (like 6" to 9" and 9" to 12" perhaps), so best to check.
Harris Bi-pod's are great quality (I've owned three, never a failure or issue) and are the original bi-pod design to first be patented if memory serves.
They are getting a little expensive from the days of old (everything seems to be increasing in value except my net income

).
However they are one of those products that remind me of a "you get what you pay for" description, but in a positive light, while holding a good re-sale value too.
Cheers D