One of the most basic ways of modeling the cell is to consider its simplest parts --- a cell wall, a nucleus and the "organelles" that perform specific tasks within the cell. You can use objects around the home to create the model --- clay and brightly colored felt, for example, can be used to replicate different organelles and help tell them apart. These models can be as simple or as complex as you need them to be for illustration purposes; younger children may only need to know about the nucleus and mitochondria, while older students might be interested in how the different organelles help with protein folding.
Another way to construct models of cells is by considering them in metaphors. Consider real-world objects and entities that replicate the functions of a cell. A city, for example, has a transportation system that is metaphorically similar to how the endoplasmic reticulum or vesicular-tubular clusters of a cell help move proteins. A submarine, with its pressurized walls, is also similar to a cell: its engines provide a useful analogy for a cell's energy-generating mitochondria, and its command center, where orders are processed and stored, can be viewed as a larger version of the DNA and RNA used by a cell.
You can also consider modeling how different cells interact and work together, rather than just taking one cell in isolation. Cells that rely on connections to other cells, like neurons, are particularly good for this. Pipe cleaners (or chenille stems) can be bent to show how the dendrites and axons of neurons connect to one another --- consider using one pipe cleaner of a different color than the others to show how a signal travels between neurons.
Home science projects can also model cells to show how they are different from and similar to one another. Modeling both animal cells and plant cells, for instance, can explore how plant cells use chloroplasts to generate energy, and how their relatively rigid walls help give structure to a plant. You can even compare the cells of a larger organism with the structure and appearance of a single-celled organism like an amoeba --- illustrating how, despite their outward differences, most cells have the same basic parts and those parts retain the same general function.