Did I overreact? (toddler biting question.)?

That 18 month old should no longer be in the room with the infants! The toddler has far too much energy, and is capable of serious injury to an infant at this point. I would go speak with the director about your concerns. They may be hesitating to move the toddler only because the toddler room is full. Which is to say, they are overcrowded. If they do not make an attempt to rectify the situation, I would look into alternate daycare. My daughter has been at daycare for some time, and she has been bitten, but the day care has a policy about how many times a child can bite before they are kicked out. (Unfortunately, it is 3 times, but it really is a part of learning for children). I will not allow them to leave my 18 month old toddler in with the babies, because I am aware of how much bigger she is than those little ones. They know that they need to let me know if for some reason they are overcrowded on a certain day. Best of luck to you!