One of the big hurdles for a PLAB candidate is convincing UKVI that the candidate will leave the UK after the examination. To that end, UKVI will examine the strength of the candidate’s ties to the home country. The stronger the ties, the more convincing that the candidate would indeed depart the UK as the visa requires.
(UKVI will also assess other aspects of your application, including money and employment. You haven’t asked about those things. You can find information here about them by searching for “PLAB” in the “Search on Travel…” box at the top of the page.)
Leaving the child behind is a tie to the home country, but having a spouse also remain in the home country with the child would be a stronger connection. The second option will be more expensive, but more likely to be successful.