Take Your Temperature
For the really little kids in the house, science experiments involving words like "polymers" or "molecular crystal structure" aren't going to go over so well. But there are some really basic science lessons that even the pre-K kids can get involved with.
Send them to collect some snow from outside, and ask them to predict how much liquid will come from the frozen stuff once it's melted. Put one cup in the sun and another in the shade; ask which will melt first. Explain the idea of a hypothesis, experiment and results. Ask them to hypothesize if the water will turn back into snow if you put it outside. Conduct the experiment to test your hypothesis, and then ask what the results and conclusion are.