Here is the Hagada I printed out this year. There's not much to it. I made it so they can color in the Hebrew letters. I made some spaces for them to practice writing (script or print) and left room to color or to cut things out and paste them. We use construction paper to cut matzas and karpas and maror etc. I'm doing this because I've been hired to teach it. For my own kids, I do nothing to prepare for Seder. (Nothing with them [unless they request]. I do think about each child on his or her level and how to engage them best at the Seder.)
As a rule, Ari prefers that our preschool children walk into the Seder knowing nothing or as little as possible. He prefers to do as much teaching as possible that night, and if they don't know the story yet, he wants it to be for the first time at the Seder. He doesn't want them to know the order of what is coming. He wants the strange order and strange things to be a surprise. He wants those things to get the children to ask, to engage them, to get them interested and questioning.
He does not want the Seder to be about children telling what they learned. He wants to do "והגדת לבנך ביום ההוא," to tell his children on that day the story of what happened.