Yo... size does matter (when it comes to snowboarding). Like stated before, if you're gonna ride powder and bomb hills, you're going to want a longer board. If you're mostly park, you'll want a shorter board. Also, you have to watch with weight. I'm 5'4 and about 155lbs and I ride a 152.. and just about mostly park (with the occasional glory ride). I've talked to numerous people and they said a 148 I believe it was, was not too practical being my weight was a little more.
You seem to be just about my size, so I would definitely suggest a 151-152. It really comes down to preference too and what you feel is right. I will use my board for example.. I ride a 152 Rome Agent... if you go to their website.. they recommend that the weight for a 149 is between 85-140lbs and a 152 is between 110-165lbs.
I am 165 lbs and 6'1" and ride a 150. It's not bad and two weekends ago was playing around on a 145. You will sink more in power but most places in the east do not have power. Most trails are pritty packed unless you head out in the woods off trail. However, if your just a partimer or weekender I wouldn't reccomend it uless you really have the experience.
Have fun though its a great time. Spent a few seasons working at Mt. Snow in Vermont.
Wouldn't a kids board be too narrow. I always use the upper lip method. if the tip of the board comes to mouth level, it's good to go. It also depends on what you ride. I ride alot of halfpipe and prefer my boards a bit longer. I remember the super short board craze of the mid 90s and they were way too unstable for normal riding.