Over-charged by Dental office?

Any suggestion what to do to fight with my kids' dental office?
Just found out that actually my 2 kids have got over-charged for cleaning since last year.
1 year ago, our dental insurance plan got changed, but the dental office did not tell us that they become out-of-network. We did not know it and assume it's still in the network, then there were extra charges ($10 for last year, and $55 for this year) My hu*****and did not ask but paid them every time except the last time ($22).

When I called the offcie to ask about the $55 charge, and then the office told me it's due to the fact that it's out of network...

The dental office does not want to give us discount on the $22 -- cleaning for my 3-year old, claims if they do it, then it's not fair to their other out-of-network patients...

I agree $55 is not a lot of money, but do not feel fair too since they have never told us. Some of the local out-of-network dentists have no charge for cleaning...
Any advice, please.