I understand that the sentence...
You can give a small boy, who is less than 1 year old, a kiss.
... is correct, but I often feel like wanting to do the following construction:
A small boy, who is less than 1 year old, you can give a kiss.
The structure fits my intuition (but this might be influence from my native language), but the sound is odd in my ears. Is the sentence correct? And if yes, are there any differences or otherwise preference towards one version over the other?
A small boy, who is less than 1 year old, you can give a kiss.
To my ear, this is grammatically correct. Standing alone, it sounds a little quirky. It might sound more natural in context.
It's pretty common in English to "front" a direct object, moving it to the front of a sentence without changing anything else. This is not the same thing as passive voice.
I kept the best pictures. I threw away the rest. (ok, active)
I kept the best pictures. The rest were thrown away. (ok, passive)
I kept the best pictures. The rest I threw away. (ok, fronted direct object)
This emphasizes the object a bit, often for contrast with the preceding sentence. That's why I think your example might make more sense in context. If the preceding sentence is something like "Greeting a male friend with a kiss is almost unheard of in America" then I think it's definitely OK.
However... fronting an indirect object (like the small boy) is sometimes ungrammatical.
You can probably fool most of them. Her I would tell the truth. (ok, perhaps slightly iffy)
For math and science, Wikipedia is too handy to ignore. An article about a recent news story I might give less credence. (very iffy)
I haven't seen the Joneses in ages. The Smiths I made dinner on Thursday. (wrong)
I don't know what the rules are. I think your sentence in particular works fine, but others obviously disagree.
You need a modification to avoid the passive:
To a small boy, who is less than 1 year old, a kiss may be given.
NOTE: Corrected per comment, thanks Jason Orendorff.
After re-reading my previous answer, and reading Jason Orendorff's answer, I realised that he is correct and mine was wrong. However, I would like to add a bit more detail to Jason's answer to explain why your sentence "sounds wrong" to me (although I'm not sure whether it's necessarily grammatically incorrect).
When forming a sentence with a verb [VB] that takes two objects (a direct one [DO] and an indirect one [IO]) in the active voice with the subject [SU] first, there are two ways of phrasing it:
[SU] [VB] [IO] [DO]
[You] [can give] [a small boy] [a kiss].
[SU] [VB] [DO] [preposition] [IO]
[You] [can give] [a kiss] [to] [a small boy].
If the indirect object is fronted, it sounds odd to me if the preposition is left out, as the rest of the sentence is in the order [SU] [VB] [DO]. I think, the sentence would sound better (although still be a bit odd-sounding) if the preposition was included:
To a small boy, you can give a kiss.
A couple of other points:
The inclusion of the relative clause doesn't make any difference to where each element of the sentence can be placed. The only restriction is on where the relative clause can go. It must remain with element it is modifying, so it has to move with that element (which you did in your example).
I imagine that the relative clause "who is less than one year old" is a restrictive relative clause, because you are talking generically (using the indefinite article "a"). I assume you are restricting the boys that may be given kisses to those that are less than one year old, rather than explaining that the boy who can be kissed happens to be less under one. In this case, the commas should not be included in your sentence:
You can give a small boy who is less than 1 year old a kiss.
and
To a small boy who is less than 1 year old, you can give a kiss.
Technically, yes. But a language's sentence structure plays a big part in how information is conveyed. The English of yesterday (think Shakespeare) is no less English than the English of today. You can certainly frame it the way you did, but it may simply be in better taste to stick with the common, modern vernacular. I'm actually prone to transposing words myself, due to my own native language :)
