how to trigger build in jenkins when changes committed to submodule?
When a git repository has a submodule it points to a specific commit in that submodule (say commit A). So even if the submodule changes and now has commit B as a child of A your top level repository is still pointing at commit A. You must explicitly update your top level repository to point at commit B in the submodule, it will not happen automatically.
Given this, the answer to your question is, just update your top level repository to point to the new commit B. This will cause a change in your repository which should trigger a Jenkins build just like it normally would, this will pick up the new commit B from the submodule.
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Given the extra information I would add a Jenkins job that watches the submodule repositories. When a submodule changes do whatever submodule specific test you have then as a post build step trigger the main repository job.