Trek 3700 XC Hardtail - looking to convert to temp road bike
If you want to use a mountain bike on the road, you put on slick tires (this is all I'd recommend doing for bang for your buck). 

I wouldn't recommend changing out the wheel sets, since this is pricey and doesn't give you a ton of gains unless you're really at the top. 

A rigid fork would be a bit lighter, but you do need to match it to the type of fork you have there (this would be a good thing to consult with your LBS on, given that you need to get the fork cut and what not). I don't think this is strictly necessary (weight isn't that much of a killer, and this is a bit pricey for a bike that the kid is going to outgrow in a few months) - just lock out the fork (if you can) and move on with your life. It isn't ideal, but it wont make or break you. 

Most importantly is to probably fit the kid as best as possible to the bike you have. 

Also, 31 inch inseam seems high for a 5'2" kid.