A few things to consider:
1) What exactly is your concern? Are you worried that she cannot successfully care for kids of different ages? If so is it at the level of a safety issue (not able to supervise toddler while caring for baby, e.g.), or just that it is not the best care (e.g., toddler is not getting out to activities? Or are you worried that nanny is stressed and might quit? What specifically is going poorly?
2) Has she handled two of different ages before? It is far tougher than twins/shares because there is no down time ever, and you have to constantly juggle. If she has only done singletons and shares then there will be a learning curve and you need to sit her down and come up with a plan.
3) How much infant experience does she have? Some people aren’t great with babies. If she is struggling eith basic aspects of infant care (getting baby to take a bottle, burping successfully, managing naps, diapering), then she may be causing her time with the baby to be tough even if he is an easy baby otherwise (baby may be hungry or tired and therefore cranky, baby might be spitting up or having diaper blowouts if she is bad at burping or diapering, etc.).
4) How committed are you to this nanny? Is the loyalty just there because she cares about your older DC, or is this someone your truly trust and respect?