I guess it depends on how far her room is from yours, do you sleep with your bedroom doors open or closed and where does the 3 year old sleep? In the same room with the baby? IF the 3 year old shared the room with the infant, I most certainly would sleep with the monitor on...on loud. Why? Because I would never ever trust a toddler to be left alone with my baby, even when they are siblings. I've had first hand experience with my husband leaving our toddler alone with our infant for as long as it took for him to use the bathroom and our first born attacked the baby. In just a matter of minutes, my infant's face was scratched. Yes, she drew blood with skin still under her nails. It was horrible so if the two of your children sleep in the same room, leave the monitor on. If they are in separate rooms, still, you need to be cautious. Can you hear when your toddler wakes up and God forbid, went into the baby's room? Would you hear anything?? You could always turn the monitor to very low so it doesn't disturb you much, but I guess it's your call in the end.