Would you take gas money in this situation? (details inside)

My kids took a job watering for a co-worker of mine while she and her husband were on vacation. They got the job because my co-worker knew I had kids and asked if they would be interested in earning some money – two of them wanted to do it. The house is about five miles from our house and I drove them back and forth every other day for two weeks in order for them to get the job done. They were both paid very generously for their work. Some have suggested I take gas money off the top of their payment for providing transportation. Others say that is a pretty “hard ass” attitude. If it matters they are both in middle school, so they aren’t little kids. Thoughts?