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.
To a small boy who is less than 1 year old, you can give a kiss.