We have Verizon for me, my DH (bberry) and 2 of my children. We do get the $1.99 on the kids phones from time to time. Here's what I learned:
If you have a game downloaded, they will charge if you add extra 'levels' that did not originally come with the game. Similarly, if you have like guitar hero (or something similar, not sure if it's called the same on the phone), it comes with certain songs and any add'l songs added gets the MB usage added.
Ringtones sometimes cost MB depending where they come from.
Check with any social networking you have that will give you updates via your phone - it may be an extra MB fee.
I suspect the $1.99 are not fake charges (I used to think they were), but hidden charges that we're not aware of because we don't think twice about how we use our phone. What I tell my kids is that just because you CAN access something on the phone, doesn't mean it's free.