No don't return the rock. I used to love getting a small peice of coral attached to a rock that was carrying "extras". I was keeping a reef tank so I welcomed any extras, except the ocassional bristleworm.
With that said if you are keeping a fish only sw tank you might not want some of the bonus items that get attached to the rock. Some people recommend putting the rock into a bucket of SW and some cirulation, no light and letting all the live stuff on the rocks die off before putting the rock into the tank and letting all the beneficial bacteria and critter regrow. I only did this once, the first time I tried a SW tank. I don't think it's neccesary, especially with any tank that is not fish only.
If you can keep the kids from buying another fish for at least a week or two, I would try to do that. You could buy 1 cheapy like a damsel maybe a couple hermits and snails and see how they do. If they survive a week or 2 then move forward with more fish.
SW can be a very expensive, time consuming but rewarding hobby. IME with several reef and fish only tanks in the past 10 years I think you should try to start slowly with any new tank.
Good luck!