Friendliest for children is hard to say, there are good and bad in every breed. Some of the most gentle breeds i hear of commonly would be - Silkies, Australorps, Seramas, Dutch, Rose Comb, Black Australorps, d'Uccle's, Wyandottes, Sumatra, and old english game bantams.
Some of the friendliest birds i've met personally, would be a black rose comb hen i had, black australorps and silkies. My OEGB's are great little guys, but some can be testy too though not heard of very often. I have to say though, Shamos are the gentlest birds i have ever seen, the stag (cockerel) i recently got will follow me around, let me reach down and pick him up, hold him as much as i want, and even reach down and pet him while he just stands there.. Never bites or anything and doesn't even know me that well yet honestly.
I'd say let your child (or if you're a child?) look at a few breeds and see what they like best, because you can get a gentle breed but if it's not one they really like or one they prefer other breeds over then they probably wouldn't enjoy it as much.. Everyone has their tastes, like i prefer games because of their personalities, people prefer silkies for the fluffiness and gentle nature, people prefer heavy/heritage breeds for their egg laying abilities etc etc..
Most of the breeds i listed can be found in bantam forms, so birds like the Shamos if you happened to be interested in them, or birds like the Australorps, Orpingtons, Cochins etc. That you would prefer a bantam instead of a large fowl because of how big they can be and difficult to handle for small children, look into them. Do your research so you can know the breeds you want through-and-through.
-Daniel