You only need 1 bottle it probably will last for a year. Read the information on Jim Humble's site and download the first part of the book, it is free.
Yes it can be used for children too, but you really should read up on it, it goes by weight. The best thing to do is to download the 2nd part of the book too, but there is fee for that.
Johny Apple Bomb has posted some information of the 2nd part so you could check his posts.
In case you don't know how, click on one of his posts and in the bottom part it will say all his messages on this forum.
There are a few answers also on the FAQ at top of page. Also check the R's.