Answer:
Fernando los cuida.
Step-by-step explanation:
Step 1: Find what the D.O.P. is going to replace. In this case the D.O.P. will replace 'los niños'. Think of in English. "Fernando takes care of the kids" -> "Fernando takes care of THEM". 'Them' is the D.O.P. which replaces 'the kids'. 'Los niños' is masculine and plural, and the D.O.P. for a plural masculine noun is 'los'.
Step 2: Now we conjugate the verb. Who takes care of them? Fernando. Therefore Fernando is the subject, and we conjugate 'cuidar' as such. The third person singular conjugation of 'cuidar' is 'cuida'.
Step 3: Finally, we put it all together. Remember: the D.O.P. ALWAYS goes between the subject and the conjugated verb (the only exception is a positive command). Therefore, the answer is Fernando los cuida.