Like Stacy said, first ask them why and to give a response that would state legally why you can't and then I would tell them to go fly a kite!!
Are you on the Board of Director's of the Little League Organization? If you are do they think that you are crossing the boundary's of that position?
It is not like you set up a scrapbooking stand in the outfield and are promoting and creating these onsite at a public facility!!
I would guess they are just sore they didn't think of something this cool themselves to make money from!
As far as taking money/sales away from the patron book, I am sure you are not doing this for every child & parents in the Little League!! That also is a parents choice to either buy or not buy the book put out by the league.