It's all entirely speculation but I'd have to get they'd just take them to the train platform in Hogsmeade near the time in which the Hogwarts Express arrives, that way everything goes as normal from then on. Luggage taken care of and they arrive by Threstal carriage or boat depending on their year.

This is all contradicted by Pottermore however which states: 

>Many pure-blood families were outraged at the idea of their children using Muggle transport, which they claimed was unsafe, insanitary and demeaning; however, as the Ministry decreed that students **either rode the train or did not attend school**, the objections were swiftly silenced.<br><br>
*Pottermore - [The Hogwarts Express][1]*

We do not know how relaxed this law can be though, as we know both Harry and Ron attended school by Car in their second year and did attend school.


  [1]: http://www.pottermore.com/writing-by-jk-rowling/the-hogwarts-express