It's hard for small children to understand the past and the future, they have so little experience with either! But one thing that is guaranteed to help them think about the concept of time in a hands-on and fun way, is through a fun defunct technology themed Science Center.
It's usually not hard to find obsolete appliances and technology. I found an old typewriter at a thrift store for $5 and I can usually find old audio equipment and keyboards through public surplus. Children love to play with these old gizmos, and they give them a physical representation about how things can change over time. This can also scaffold their ability to think about the future. If people used to use a typewriter to write letters, and now they send messages on their phones, what might we use to send messages in the future?