Costs money

1. Conventional print advertising in newspaper sections or magazines that target people with children
2. Facebook ads (target parents or w/e)
3. Google ads

Doesn't cost money

1. Community noticeboards at Coffeeshops
2. adding inserts to free newspaper and classifieds -such as the green sheet or that Spanish car one
3. hotels with banks of brochures for visitors- drop your own brochures in one of the compartments
4. Staple disposable, biodegradeable, removable fliers (read: paper) onto telephone poles
5. public transit vehicles with compartments for fliers or maps? Do just like the hotel and leave your own brochures.

This really isn't hard… Just don't vandalize other people's property. Graffiti is a very low cost, low effort, low-repute means of spreading a message, at least if one wants that message to be a mainstream thing. Someone looking for low-cost, low effort ways to contribute might want to rethink whether they really want to contribute at all.