It is Humane to keep Chickens cooped, Most do it to keep them safe.
You should have 4 sq. ft.
(or more) per bird in the coop & 10 sq. ft.
(or more) in the run.
If you have things for them to do like a hanging toy with some greens & a bell or a small pan with sand in it, lots of roosts of many sizes & heights ect. They will be happy, even more so if that's all they have ever known.
Just keep them well fed
(which makes them happiest), Dry & in a draft free coop & they will pay you back in eggs & if you put enough time in to them when they are young Chickens make darn good pets. It's all in how you raise them.
Plus you can always let them out to free range if you are watching or before you reseed your garden, They will help you till the land & a small amount of Chicken poop is good for the soil, any more though & you will have to compost it.
As far as your son & the poop... chances are you won't have enough Chickens for that to be to much of an issue, If you are that worried just keep them in their Run/Coop most the time & when you do free range them just hose down your lawn after.
If you plan on keeping them penned up most the time what ever you do don't get the breeds listed as
"Flighty," Some of these breeds are:
Leghorns, Lakenvelders
(my favorite breed), Hamburgs
(Freda a silver spangled Hamburg of mine like to roost 220' up in a tree at night...Some breeds of Chickens can fly for short flights. She got the name freda b/c she wanted to be free or more like Human free...lol)
Some say Polish are flighty but I haven't had an issue with them.
Generally speaking long bodied birds are flightier then compact birds
My sweetest calmest breeds were my Bantam Faverolles, Dorkings, also Ameraucanas
(not Easter Eggers!) Dominiques, Langshan
(My 2nd favorite breed), Delawares & Serama are just out right pets, not good for eggs & neither flighty or not, but they were bred for no other reason to be pets & they make mighty good ones.
Some people say Cochin are a very tame breed as with Silkies, but in my opinion it depends more on the line of those 2 breeds rather then the breed them self.
Let me ask you some questions that can help us help you:
So why do you want Chickens?
How much land do you have?
Can you have a Rooster?
How many eggs dose your family go through in a week?
How many birds do you want?
Where do you live: How Hot/Cold dose it get?
Do you care about the breed(s) you get?
Do you want your Chickens to look the same or to be able to tell them apart?
Do you want Chicks or are Started & Adult birds okay?
Do you want them to be pets or just to lay eggs?
Are you going to eat them?
How much time a week to you have to care for them?
What size would you like them to be?