Well, we thought problem builder was the only XBlock using this capability, and we have started the process of planning a migration strategy for it: Future Directions for Problem Builder
We can certainly make recommendations but they would likely be what I just said above: moving from multiple child blocks to one XBlock with a more complex data structure to represent it. We also hope to have an improved XBlock v2 API that offers more simplicity and flexibility to XBlock authors, but haven’t made much progress on that yet.
There are also some built-in XBlocks like A/B testing, randomization, etc. that use XBlock children but aren’t using StudioContainerWithNestedXBlocksMixin and there is a migration plan being developed for them - moving their logic into a new “selector” API at the unit level.