start with finding out how much money your demographic (buying children) have to spend.
Don t reinvent the wheel. Take a look on the web and find other similar businesses. Start with their business plans. Even their table of contents will give you some idea of what problems you may encounter. Remember you don t have to have an exact match. A second hand clothing store, a consignment store, a normal furniture store business plan will give you some ideas. Good luck.
Go to a Junior College in your area and take a course in Small Business Department.