Can I claim my child as a dependent if they work?

OK I know this is a silly question but hear me out.  My son is 9 and I want to write him a check for doing yardwork and put the wages into a Roth IRA account for him.  Now he will only be getting paid $25/month for the doing the yardwork.  I guess there are a couple of parts to my question.  How do I document it on my taxes that I employed him to do such work?  Will he have to pay taxes on these earned wages, will he have to file taxes for making $300 for the year? Should I withhold anything from his check. Like if I am paying him $25 for the work should I only write the check for $20? What records do I need to keep? Any information will be appreciated.