Welcome to the Cove!
Hi, Tracey!
Like Dave B said, it is up to your Organization to determine what constitutes a "qualified" and "competent" auditor.
When I first started at my current position - just over one year ago - I was appalled at the state of the Internal Audit system! These were people who had taken the 2-day off-site course and deemed "qualified". The caliber of their findings (or lack thereof in some cases) was shocking for me to see.
I can not say who should train your Internal Auditors...only your organization can determine that. You need to sit down and think about what the purpose is of an Internal Audit and how your organization benefits from them.
You should think about what sorts of results you hope to gain from Internal Audits, too. In my case, I enjoy receiving findings from an Internal Audit - it means there's one less thing for my External Auditor to harp on. I am pleased when my auditors assign me an Opportunity for Improvement - they see ways of doing things that I have yet to think about.
Training Internal Auditors, in my opinion, involves more than just being able to parrot off the Standard. You should know the audit process (especially how it applies to you), you should know auditing techniques, you should be aware of potential roadblocks and how to overcome them.
If you feel that your audits are providing a benefit to the organization, why shouldn't you be "competent" to train your Internal Auditors?
Out of curiousity (I noticed your company size is 50), how many auditors do you want? I tend to compare my Internal Auditors to a stable of thoroughbreds. I would rather a few excellent horses, than a whole herd of mediocre ones.

(granted, they don't usually enjoy being compared to horses)