Some dotted square or triangular paper would be useful here. I found old fashioned curtain rings to be really good. I guess that coins would be useful!
This lends itself to a lots of number pattern work, algebra and shape work.
Beware of the problem when children are counting or communicating generally about what they've done because of the tendency to count the number on each side inaccurately. In this example:-



There is the confusion of counting 4 on each side and then multiplying by 6, instead of counting the "new ones" each time and saying that there are 3 new ones on each side.
There is a lot of opportunity for symmetry work to be done as well as multiples and factors.
Just let them enjoy this and see what opportunities arise.