objective lense

I am looking to buy a couple of objectives for a Spencer 60 by
American Optical, My Kids have discovered the microscopic aspect of
the world and want to explore a little. I currently have a 10x and a
40x and would like to get something a little stronger to get up close
to the little critters swimming in the ponds and streams around us.
Any suggestions about the best magnifications to use and sources will
be greatly appreciated. Phil