Cuisenaire Rods and Base Ten blocks can be used to represent variables in algebra. This makes it less abstract and young kids are able to understand what would otherwise be completely beyond their math and logic abilities.
My girls have had previous experience with square numbers and I think that helped them to catch on to this quickly. You can see videos of them playing with square numbers in base ten here...
I learned this "x squared" stuff from Crewton Ramone's House of Math - this video specifically… youtube.com/watch?v=4T5-Iu64e6k&feature=relmfu He knows a lot more than me so go check out his videos.