The new GT Aggressor sold by Dick's Sporting Goods is a solid entry level MTB. Definitely a step above what you'll find at Wal-Mart etc. I recently bought one for one of my kids and I have no issues with it. Good disc brakes, modern geometry, and mostly Shimano components. Watch the sales, I got mine for $299. It's $399 right now.
https://www.dickssportinggoods.com/p...ggrssrprxxxprf
It's geared perfect for pulling your trailer too.
I ordered mine online and had it shipped. Shipping was free and it was fast, about a week. It arrived in perfect condition. It does need minimal assembly, but nothing too complicated.