A 10 yr old is old enough to play with N gauge.. and you can do a LOT on a small layout that will slide under the bed in N gauge.
BUT if you still want HO, buy locos from Lifelike, Athearn, Atlas. DO NOT BUY BACHMANN. At one time Lifelike was junk, but they have come out with some really nice products the last few years. Pass on their train sets.
www.atlasrr.comwww.athearn.comwww.woodlandscenics.com/home.htm (this is the correct link, but not working now for some reason)
Get the basic layout book from Atlas. They have a lot of small bedroom layouts.
Best bang for the buck in cars is Athearn. They are "shake the box" kits. Well painted, good, free rolling trucks, nicely detailed, inexpensive.
MRC power pack. The MRC Railpower 1370 or Tech 4 200 is hard to beat for a basic power pack.
For a small bedroom layout, a small "road switcher" such as a GP30, GP38, GP40, etc, will serve as a yard switcher or road unit, just as the do in real life.
Woodland Scenics... They have a little how-to booklet available, get that first.
For kids, avoid "trestles". They may want an "up and over" figure 8, but this is asking for trouble.
HO sectional track must be nailed down. Again, the Atlas book will tell you how to do this. It is not like the old Lionel stuff that could be plugged together on the floor and played with, then taken apart and tossed back in a box and shoved under the bed.
In N scale, Atlas, Kato, LifeLike, Athearn are all good lines.
In either scale, at first, avoid the temptation to load up ("collect") a lot of cars and locos. Pick out a nice, quality loco that will run well, and half a dozen nice cars. Think about what kinds of businesses, industries your cars will serve. Look for buildings that can represent those businesses. Perhaps model a type of industry and the railroad that serves it that is in your area. The scenicing of a model railroad is what really makes it a lot of fun and impresses visitors.
Have fun!