Yes my sister and I used to talk like small children to each other, and to Dad, when I was about 15. I think Dad had started it by mimicking a child we'd seen on a TV documentary, who was kind of wiry and cute and pronounced "S" as "F." In the end we were talking like that to each other for most of the time. Dad usually encouraged us, but Mum didn't approve and would never have lowered herself to join in. It felt like a strong bonding force between us, probably because it was the only remotely creative thing we shared with each other.
I have reason to believe that my sister and her hubbie continue the tradition occasionally in private. It was certainly happening before they got married, and he found it amusing.
I was a little surprised when my first serious girlfriend began talking to me like a child, without ever having known of my family tradition in that respect. I found it easy to reciprocate, in fact I did it so much that she started getting annoyed about it, and we stopped.
It's happened with one or two other partners, though it's not what it was......with my current wife I don't, but I've often noticed that what I say to her sometimes does sound childlike, not very noticeably so but consistent with the words coming from an intelligent ten-year old.
I don't know what it all means. I've read that it's common for couples to talk like children to each other.....I think the proposed reason was something to do with a psychological regression into childhood that was said to affect couples.
I did notice that it was often very difficult to break out of it and talk normally again. Sometimes I wondered if that was down to some kind of inferiority feeling, being reluctant to speak in my own voice simply because it was mine, and therefore no good. Like my sister and father, I like to adopt the voices of other people during otherwise normal conversation, apparently for comic effect, but I don't get stuck with those like I have with the child voice.