What s the youngest age you get them to walk?

[torontokids]

I currently have an 18 mos old (mine), 15 mos old, 16 mos old, 3.5 y o (mine), 3.5 yo. I have a spot for Sept that I have been trying to fill. I have turned down a few families with infants because I don't think I am ready for another little one. I would find the program/inside stuff fine but the traveling, walking, park thing scares me with so many little ones. A larger stroller isn't an option (have a triple, it folds up) as I don't have the storage for a runabout etc. Currently the 3 littles ride and the big kids walk. Anyone start having younger 18 mos as a walker? Just trying to figure out logistics.

Thanks!