What do you call the child who doesn’t resemble his / her parents in English?

Usually it would be polite not to mention the fact that a child doesn't seem to resemble its parents, since this might imply infidelity on the part of the mother (not good for Susan George in the 1975 film Mandingo). But you will sometimes encounter OED's sense 6a for...

sport - a plant (or part of a plant), animal, etc., which exhibits abnormal or striking variation from the parent type, esp. in form or colour; a spontaneous mutation; a new variety produced in this way.

This noun usage derives from the (now rare) verb sense...

sport (v 8a) - of nature (originally, personified): to ‘amuse herself’ or delight in producing the variety of things in existence, especially abnormal or striking living forms; to produce such forms.

As @ronan comments, facetious "He looks like the postman / milkman / etc." is not uncommon.

There's also throwback (reversion to an earlier ancestral type or character; an example of this). But unless accompanied by a reference to a specific grandparent (or perhaps great grandparent) that the child does resemble, this invariably has negative associations (with the implicit unspecified ancestor being a Neanderthal or some other precursor to homo sapiens).

You could use the/an apple that fell far from the tree.

This is a play on the saying "The apple doesn't fall far from the tree", meaning that children tend to resemble their parents both physically as well as temperamentally, or even in more mundane aspects such as job choice or favourite football team.

By saying it this way, you leave it up to the listener (or to context) to decide where the difference between child and parent lies and whether the difference is a positive or negative one.

For the temperament side, I've seen "changeling", a reference to the medieval belief that children were sometimes stolen by fairies and replaced by imperfect magical copies. I've never heard a term for children who don't look like their parents short of technical terms such as "outlier" and, of course, the occasional term that implies that they were fathered by someone else entirely.

How about, "The baby was switched at the hospital." Or, as I like to say when asked why my brother and I don't look anything alike, "Our mothers were switched at the hospital."