Can you clarify why aluminum is not easily repairable compared to steel? While it's true that steel tubing will resist dents and bending better than equivalent-spec'd aluminum tubing, steel is subject to damage from rust. As the other commenter said, this means repairing a steel structure requires sanding off the paint layer, which isn't required at all if the natural color of aluminum stock is acceptable. And both can be welded using readily-available processes, so repairs should be substantially similar to initial construction.

As for strength, steel is indeed stronger in tensile and yield strength. But for tube segments in compression, that quality of steel isn't being exercised, since you can always use aluminum tubing with thicker walls to support your expected payload, and still be lighter than the equivalent steel tube section. 30 kg is well within what commercially-available aluminum round tubing can support, for the dimensions of the average bike rack.

What can be a possible consideration for steel vs aluminum is whether you have significant sideways forces. A payload centered above the rear axle simply follows where the front wheel is pointed. But if the payload is heavy and behind the rear axle, it swings to the outside of a curve, causing flexing in the rack.

Aluminum tubing should be designed so that sideways forces do not "collapse" the structure by allowing the tubes to be kicked out from underneath. This applies to all tubing, but steel's high stiffness compared to aluminum means steel segments can be much longer before running into these same considerations.

But I think we're still getting ahead of things. Can you describe what your current bike is, or what existing mount points it has for racks? And also perhaps a description or sketch of how the rack would be loaded? The engineering is necessarily different for a 30 kg child versus two 15 kg panniers.