it's not childish, why you think so?
You asked for a native speakers opinion. That's my opinion.
I like grandma, I like my dog, I like you, are the sort of phrases a child would use.
I like you, from an adult to another adult, can lead to a misunderstanding and hope of something more on the part of the reciever. This has been explained.
One of the problems with English language is: Not what you say, but how you say it.
I can say: 'I love you', and the other person will know that I'm just being nice.
If a foreigner says it it might not come out the right way.