I don't teach kindergarten all day, but I do for a half hour (by myself, with 18-20 students), and I also work in the kidnergarten at other times, so I can say that 1:18 is do-able in kindergarten, but a few things will likely happen.
1. The class will be more whole-group oriented. There will be less "exploratory" learning, because such things require a level of attention that just cannot be sustained for long periods of time with just one adult.
2. The need for order increases. A few people were APPALLED a while back that a teacher required that all students sit cross-legged during story time. Well, with 18 wiggly five- and six- year olds, and no other adults to help keep them, there will likely be more of a need to have everyone sitting quietly in a certain way, simply because of numbers. If you have an aide, you can put the wiggliest kid on his/her lap, and two others who require a little assistance next to them, and maybe one more right in front. They can be allowed more liberty to wiggle around because the aide is there to make sure that the wiggling is really just attentive fidgeting. Without that extra person, the teacher pretty much has to take a harder line just to keep everyone focused on the activity at hand.
3. Your daughter will get less one-on-one time. She won't get NONE, because teachers (the good ones, anyway) strive to make meaningful contact with all their students every day. But numbers dictate that she will get less. She can still get a good education, but she will get less one-on-one time.
4. Special needs may be either over- or under- recognized. The quiet, compliant, but struggling student may fly under the radar, while the capable but very active student, in an environment where there are fewer adults to redirect energy, may become a disruption and be referred for special services.
Overall, I prefer smaller ratios. In my classrooms, I aim to make them as close to a homeschooling environment as possible given the environment of a public school. The fewer adults, the further away you have to go... NOT because the teacher is deficient or uncaring, but because it's just how it works at higher levels.
Can your kid still get a good education? Yes. Would it be better with another professional in the room? Yes. I know that I do MUCH more student-focused, project-based work with my first grade groups (we break their class into two groups so I have fewer students at once) than with my kindergarten group, who I take all together. I still do meaningful and worthwhile things with the kindergartners, but it's definitely harder to do unstructured, exploratory, constructivist kinds of learning activities without a sedond adult around to help guide them.
