It is, I think, reasonable to assume that books aimed at young children will not contain obscene language. It's not a matter of "freedom of speech", but what sort of language you'd want your own children to learn is appropriate to use. Children of that age are not in a position to judge for themselves what is, or is not, good language use - how many 10 year old are going to know what the word means, do you think? Isn't that one of the reasons that we encourage children to read, to improve their command of the language?
If you buy a children's book it's perfectly reasonable to assume that it won't contain sex scenes, strong language, extreme violence, etc, IMHO.
I have no issues with the use of such language in books aimed at an adult audience, obviously.