Teaching several different ages of children at once can be difficult sometimes! Resources such as unit studies are designed to help teach multiple ages at once. All ages study the same science topic, but older students read more in-depth material and perform advanced experiments. This prepares them to answer more challenging questions and demonstrate a greater understanding of the scientific concepts. Young children, in the meantime, learn the basic concepts and perform their own simple experiments. They are usually fascinated by the experiments that their older siblings get to do, also, so you can encourage their interest in science by letting them watch (or participate in) the more advanced experiments. Have them write a simple lab report with pictures and their step-by-step procedure.
Even if you are using a traditional grade-specific curriculum rather than a unit study, you can still include your other children. Involve young children while you are doing experiments with your older ones by giving them simplified experiments similar to what your older children are doing. For example, if you are teaching junior high or high school-age children about acids and bases, have your younger students mix baking soda and vinegar to demonstrate safely how acids and bases react violently when put together. If you are doing a study on plant classification, it might be a good opportunity to teach your elementary age children about differences in plant leaves. Microscope studies are something that all ages can participate in - young children can view slides, and then draw pictures or describe what they see.
Another great way to get children of different ages involved is to take a field trip. Some ideas for places you could visit: a manufacturing facility, a farm, a state or national park, a recycling center, a planetarium, zoo, print shop, or water treatment plant.
You can also enjoy books that have vivid pictures to capture the interest of the very young and more detailed information for your older children.