Our sessions run for 8 to 10 weeks depending on the season. At the beginning of each session we provide parents with information about the program, the benefits of exposing their children to a new language at an early age, rules in the class, the importance of their active participation in class and what it means for their child's learning process. I don't use posters with lyrics because I observed that some parents were focusing their attention on trying to understand every single word and also more likely to mispronounce them.
I currently create the music and curriculum for the program because I find that many of the traditional songs don't provide enough space for repetition or have the target vocabulary that I need in context. At the beginning of the session I provide parents with a music CD that contains all the songs and games for the session along with lyrics and translation. You can also look for songs that parents can purchase to support what you do in your classes, extend the learning at home and keep those little ears and mouths listening and singing outside the class.
There are also many online resources you can create such as creating a blog where you can upload small audio files (that can actually be recorded with a small digital camera for pictures) for parents to listen to the songs you sing in class and sentences to use at home. From my experience I advise you to write the sentence in Spanish next to the audio file and to avoid using the phonetic spelling to avoid mispronunciations (especially of vowel sounds and "rr")
It is also useful to have some props such as a parachute, bubbles (to count) and objects that are colorful and appropriate for little hands to handle, e.g. finger puppets to accompany a song. Movement is also important when teaching toddlers - that's how they grow and learn. Make sure they have space to run, jump and enjoy the class, but at the same time make sure you have a transition song to bring them back to the circle. I also use ASL with some of the songs; there are many resources on the Internet to find the signs.
At the end of the class I provide a few minutes for parents and children to socialize. Parents take advantage of this time to meet other families and also to ask you how to say phrases and sentences in Spanish.