How strong are bolts compared to welding and what qualities to look for in foundations?
Unless your holes match the bolt size very precisely (like, you have to hammer the bolts home), you will get racking, which will weaken the joint over time. Likewise, the bolts will loosen up (use lock washers and check it frequently). It's something you'll have to watch for; as the holes get stretched, you'll need to figure out what to do. Think about how thin the piece of metal between the holes and the edge of the piece is; that's the weak point, strain-wise.

Also, a freestanding wall is going to be a tremendous strain on the footings, especially with someone (or a pack of neighborhood kids) scrabbling around at the top of it. A person can throw their weight around to the tune of 5x actual weight. At a distance of 10'-15' that is tens of thousands of ft-lbs on the bolts, and any weakening in the joint will make things worse and worse.

If you can arrange it, build it as an A-frame (climb the sides of the 'A') with at least four footings to reduce the mechanical advantage the climber has over the footings; think of a playground swing. Otherwise, it doesn't matter how strong the wall is, the whole thing may tip over or may be yanked down or ripped off its moorings.

Given that people's safety is at risk, you probably should consider consulting with a professional structural engineer. The strain strength of steel under sudden load is much smaller than you'd expect.