Our last pond had a fiberglass cover, fit the pond. I do not know the history of how this came about.

I couldn't follow a couple of links but think I understand what you want.

Steel would be the best (only) choice. Sometimes when steel is made into a decorative form people call it wrought iron but wrought iron stopped being made years ago. There are infinite types of steel, but all would be OK. You should probably wash the metal with soap first if being left bare metal.

Other metals would be extremely expensive and many are weak for spanning any distance.

There are temp pool fences which are common here in AZ. Way cheaper than having a custom metal cover.

You can also fill the pond with pea gravel. You can remove gravel to make the pond deeper and deeper as the children get older. They can even play in the gravel and water which is great fun for kids. Some parents are very protective and wouldn't like the contact with bacteria so that might not be an option.

You can also fill with pea gravel but build a smaller pond within the pond using concrete blocks to hold back the gravel. That way you'd have a smaller area to cover which is cheaper because it's smaller but also because the span is shorter so smaller sized members can be used. You can even build a pillar in the center to reduce the span further.

The nice thing about a fence is we have a lot of experience about what works. So to me it's safer. When you get into making custom structures there's no testing. People just look at something and think "wow, that looks strong" using their vast engineering skills. By vast I mean zero. The kids will have to do the testing. That's not too cool imo. If something bad happens after I install a pool fence to code at least I did the standard practice. If it happens because of some deal I invented and "thought" was safe...that's on me.

For example, there's code that defines the min and max openings. This is obviously to stop kids from being able to fit thru an opening but also so they can't squeeze their head thru and get stuck. That would be really bad if their head was stuck in water. Even if "safely" stuck it can't be fun to have the fire department to come cut them out.

So I would treat it as a swimming pool and check code in your area. That way you gain the benefit of many years of human experience and what was learned from many unfortunate child deaths. Better to learn from all those tragedies than start from scratch on your own trying to invent a better barrier imo.