I would say that every child job should be updated to latest template on next build.

It is a good idea, but currently it doesn't happen. Anyway, that might be a problem when the change relates to the own build, such as the labels where the job can be ran.

I've tried by adding a new build step, waiting for it to be updated and it didn't update the child jobs.

I would say that if somebody needs a customized version of the job, the proper way would be to clone it (thus breaking link with an automatically generated job) and customize it at your own liking.
The opposite is that if you have lots of branches and you need to update build job on all of them, you need to go and delete all of them and wait for the system to regenerate them with an updated template. IMO customizing stock jobs is a shady practice and thus should be discouraged, while updating build jobs is a legit desire (e.g. on a long lived projects) and should be supported out of the box.

I agree.

Maybe there is another possible scenario: to have a button to break the configuration for an specific job, just like the "main this build forever". But this is not the issue I'm asking for. As you mention, what I really really need is to have all subprojects in sync.

I patched it with a python commandline program that regenerates the job, but I'd prefer the plugin to do the task.