I find they start walking within weeks of starting at the age of 12 months.

In saying that I think you should stick with your comfort level. The 18 month old would probably be close to being able to walk short distances, but you're still a couple of months off. I would be nervous having 3 in a stroller and 3 walking beside or any age.

Then again, we stick to the back yard a lot of the time. With all of this unlicensed daycare issues in the media I try to keep a low profile.