It's traditional "baby talk," or, if you prefer, "child-directed speak" or "motherese." It's a kind of affectionate babble-like sound parents make to their children. Long before children start to form words and discover grammar, they must first learn that sounds communicate.
For some reason, "itchy kitchy koo" is a kind of standard example, just as "Hip! Hip! Hooray" is the traditional example of a cheer, or "Bow wow!" is the tradition rendering of a dog's bark.
Just making this up--a longer example might go something like this: "Ooooh woooh woooh! Ickle ickle ickle ooo my sweetie izzums, izzy? Yes he IS! Yes he IS! He IS! He IS! He IS! Is IS? He is! Izzy izzy izzy-kins! And does 'e wuv his da-da? He does, he does! Does he wike to be tickle-ickle-ickled? Tickle-ickle-ickle! Woo woo woo!"