Is Trek not making the 26" Dual Sport Kids model anymore? That seems like it would be perfect for your needs -- a kid's sized 26" wheel bike with an aluminum frame:
https://www.trekbikes.com/us/en_US/b...colorCode=grey
My 9-year old is 4'9" and on a 24" and I don't think she has much more growing room, so I think a small 26" could work.
Another good options would be the Woom 6. 26", extremely lightweight, lots of growing room. Not as cool looking as the Trek though.
https://us.woombikes.com/products/6 We've got the Woom 5 and really like it.
I'd take a higher end lighter kids bike over an entry-level heavy adult bike. Or, based on what you've described as your intended uses, I'd go with something like a Trek FX hybrid instead of a mountain bike.