Find out their termly topics (most schools will provide this information each term, or you can always ask their teacher) and take an interest — find relevant books in the library or bookshop, do some research, brush up your own knowledge about the topic! Then you can have interesting conversations where you are both learning at the same time.
Why not take a trip to a science museum, a zoo or an aquarium? These don’t necessarily need to be completely related to what they are learning about at school. Any visit can help their curiosity and engagement with science generally.
Find out about famous scientists and research unique and exciting inventions up to and including the present day. Who knows, you may have the next Stephen Hawking or Marie Curie at home!
Look up fun, practical science experiments you can do at home with everyday objects.
Anything where they can be hands-on and see the science happen in front of their eyes is guaranteed to be get them interested.