Look up Jericho, Sand Mountain, Little Sahara as these are all names for the place. It is almost straight west of Nephi utah. Drop Jericho in
www.mapquest.com and it just about puts you there. From south ogden you are about 120 miles or so.
As far as children's atvs, I know you need spark arrestors and flags. I am not sure about registration of children's ATVs but you might want to get a number for the visitor's center down there and give em a ring.
There are tons of places to go down there, I reccoment going to the white sand's area as it is a bit more family friendly than sand mountain itself which is about 8 miles away. The serious psychos go to sand mountain and camp at the base, pretty much where I am standing in that panorama picture.
There is an entry fee also, I wish I had the brochure on me, I am at work right now.