Are there ways to protect intellectual property without getting a patent?

I work for an after school program, with elementary age students. As such, I must plan various projects and activities which will both entertain and educate children. I recently designed a chemistry/science based board game, complete with question cards and a game board. Please understand - this is NOT a preexisting game which I modified - I designed and created an ORIGINAL playing board and personally thought up the 70+ questions on the game cards myself. It can be played with 2 to 6 players and is simple enough for a kindergartner to win, yet challenging enough to hold the attention and interest of a 5th grader (not an easy feat!). It has been several months since I first introduced the children to this game and they still ask to play it during free time at the program. Because it has been such a hit, I am considering possibly approaching some of the better known game making companies (Milton Bradley etc.) about buying it. Here is my question: How do I protect the design and concept from these big companies once I've presented it to them? Meaning, how can I keep them from stealing my idea and marketing it as their own? I am a single mother with very limited financial resources, thus the patent process (which is lengthy and expensive) really isn't an option for me. What other means can I use to protect my intellectual property?