I overheard today that there is a few people here at my job who are on a better dental plan. I guess it's not being offered to everyone because of the cost, and because the other's have children. Can they do this?
This was not negotiated in their salary is it a new offer.
It's actually was offered to employess without children.

I think that benefits are part of your salary negotiation. Therefore, they are confidential between you and your employer and it is up to your employer what he/she wants to offer.

At the same time, it is illegal to discriminate based on age, gender, etc.

I would recommend negotiating your benefits in the same manner in which you negotiate your salary - in a closed room discussion with your employer. Avoid gossip and office discussions about salary and benefits - right or wrong, getting in the middle of this will only reflect poorly on you to your employer.