I have extras only cover, which was MBF Classic Extras about two years ago. I pay almost $60 per month and that covers myself and all my children. However the plan is no longer offered to new customers.
A few months ago I tried looking around at other options, and was told that for the same level of cover I would be up for around $100 per month on one of the new plans. Therefore I chose to stick with what I already have.
Your 5yo should be entitled to free general dental through the school system until he is 16yo IIRC. So if it is just for him, I don't think you really need it at this point.