If I were you I'd start with things you're familiar with producing -- such as prints in standard sizes (4x6, 5x7 and 8x10). Whoever prepared the ad that you saw probably has specific requirements too so be sure and ask them what they want you to offer. You might also want to prepare a simple questionnaire that you pass out to parents in the beginning that asks what they'd like for you to offer. The friend you mentioned who'll help you with the business side might have suggestions so be sure and ask her too. Your maximum profits will probably come from things that are 1) Easiest for you to produce because you'll have the least amount of time invested in them and 2) From bulk sales because it's easier and less time consuming to make multiple copies rather than individual copies of a product.
As for order forms and pamphlets start with stuff you can print yourself. If and when business justifies it you might want to consider having these printed by a commercial printer -- you'll also have a better idea of what you want them to look like if you've already used some that you made yourself. IMO business cards are essential for almost everyone who conducts any sort of business so be sure and hand them out at venues you shoot. You can either design and print business cards yourself or have them made commercially -- I've done both.
Don't expect to make a profit in the beginning. Most businesses operate at a loss when they first start and those who survive usually offer better service and/or higher quality products than their competitors and at fair but not necessarily the least price.
