For The Parents...
Okay moms & dads, there's something you really need to know, and you're not gonna like it. If your kid is a real RC maniac, there's a 99% chance that he or she knows more about these toys than you do. A lot more. Kids see all of the RC commercials on TV. The know the significance of theme-based vehicles & craft and the fictional characters or real-life celebrities that inspire them. They know the slang terms for stunts you've never heard of. Accept these facts, embrace them, and get ready to have fun getting caught up!
The best activity you can engage in with an RC maniac kid is collaborative shopping. Get a pencil & notepad and take him down to the local toy superstore or all-in-one store that has a good toy section. See what RC's he's drawn to. Ask him what he likes about them and take notes along with brands & model names and prices. Don't buy anything just yet! Now take him home and look for a review of the same or similar toys here on RC Mania. Browse online stores like ToysRUs.com and eToys.com with him, and pay particular attention to user-submitted reviews from other kids and parents. Once you're sure of what he wants, do some comparison shopping for the best prices, paying attention to taxes (where applicable) as well as shipping charges. Don't forget to check eBay for surprising deals on both lightly used and brand new-in-box products from individuals and stores alike.
Going it Alone
When it comes to gift-giving, if you value a good surprise (as opposed to asking your child to tell you what to buy), you're going to need to work a little harder. Pay attention to what RC commercials on TV your child responds to the most. In the holiday season, search for "hot toys" lists released by magazines and independent panels. Take a look at the toys he already has and likes, as opposed to what ones have fallen into disuse -- see if you can discern a trend, and get ahead of it!
If your child raves about one particular RC toy all the time like he's got to have it, research it first. If it actually performs poorly, sure your child will be ecstatic when he first gets the gift, but it'll only be a matter of hours (if not minutes) before all of that enthusiasm converts into heartbreaking disappointment. If the product gets good reviews, but is out of your budget range, try to find something cheaper that has a similar look but doesn't skimp on performance (especially speed if you're talking about a car, truck, or boat, and ease of flight if it's a plane or helicopter).
Toy-Class vs. Hobby-Class
Once you start doing some online research, you will undoubtedly discover the expansive but little-known world of "hobby-class" or "hobby-level" RCs. In that world you will find everything from tiny 1/28th scale cars to giant 1/4th scale airplanes and everything in between. You'll see built-it-yourself kits that take weeks to assemble, and you'll see ready-to-run sets that are more complete than many "toy" RCs. A nice summary of some of the differences between toy- and hobby-class RCs can be found right here (two pages and a good site to browse around). Is one intrinsically better than the other? Absolutely not, but "fun" is a relative term, so keep in mind the age and mechanical aptitude of the person you're buying for.