What is the perfect age for flower girls?

I'm asking this because a friend is getting married really soon (as in 3 weeks soon!) and we as her bestest friends are in a panic preparing the details for the wedding. This question popped in my mind while doing the measurements for the flower girl dresses that my friend chose for her entourage. The youngest of the flower girls is 21 months old and the oldest is 5 years old. I wonder if the 21 months old cutie pie will be able to walk the aisle without so much a cry. In my opinion the age bracket should be 3 to 5 years old. Even at that age, tantrum is very very unpredictable. And my friend specifically said she wanted a peaceful and solemn church wedding!

What to do???

Few things come up my mind:

Maybe I can ask the mother of the younger ones to stay on the front and motion to their kids to walk to them. This way, kids will tend to focus on getting to their Mom and voila! walks the aisle with no trouble.

Or, I can ask the Moms to accompany the cutie pies while walking the aisle..but that won't look good huh?

Well, Sept. 8 is pretty near so I'll just have to tell you how it went :)

Or maybe you have better ideas?