backyard...

If you are asking will they poop where they forage yes! Will it be a huge mess, not really, as you know chickens make small droppings (relatively speaking). But you can bet your children step, fall or roll in it (not on purpose of course).
Has any child ever died from chicken poop? If that where the case most of us on this board would not be on the sunnyside of the earth!
As far as roaming that is a chicken specific thing. I have hens that don't go no more than 100 inches from the coop when allowed to roam, others can be found 1000' feet from the coop. Trying to catch a chicken in a open pasture can be a difficult thing to do and probably not practical.
As you know predators are probably the biggest concern in letting the chickens free range. I always just let mine out about a hour before dusk, this has a tendancy to keep them close to the coop and usually past the dinner time of birds of prey and before the dinner time of nocturnal predators.
Hoped I helped a little.

 

I would worry about birds of prey picking off one of your chickens.

A member from Vancouver,WA - Lisa something just had that happen to her last weekend.

She would let her chicks outside the run when she was out in the yard keeping an eye out overhead and just when she stepped in the house for a minute, a hawk came in and caught Lady B at the gate of the run. So far, she has survived because she ran out and yelled and the hawk flew off before taking her head off.

I know plenty of people let their chicks free range but I wouldn't chance it.

I'll be building my girls a tractor so they can safely eat fresh grass in my huge yard and 4 acres.