By default, submodules are initialized in a detached-head state and not made to track specific branches, even when you specify a branch when initially adding the submodule. This means that any commits you produce will not be on a particular branch and the head commit will not be updated to point to new commits (you would not be able to push any new commits, at least not in the way you expect). This is fine where there is no active development, but, otherwise, you would likely need to intervene and individually checkout each project to the branches.
Assuming you specified a branch when you added the submodule, you can use the “git submodule foreach” subcommand to automate this:
git submodule foreach --recursive 'git checkout $(git config -f .gitmodules --get submodule.$name.branch)'
You can run this from your supermodule project or qualify the “.gitmodule” filename with its path.
If you need something more complicated, you can obviously write a script and call it from this context.