Marlin no longer shows the 35 as an option on their website. If I wanted one I'd get it now. If they have discontinued them prices on used ones will bring a premium soon.
I've used the 35 and 30-30 a lot over the years. I cannot tell any real difference in recoil although I'm sure the 35 has slightly more, at least on paper. Cannot tell any real difference in performance either although some guys swear the 35 is a better deer killer.
Can a 9 Year old handle it, probably. But it depends a lot on the individual person. While neither 30-30 or 35 have a lot of recoil I've always felt the stock designs on the lever guns tend to make recoil feel worse than it should. I'd start a kid that young on a 243. But if a 35 is what you have and you don't want to spend money on another gun I'd give it a try.