Another math circle activity that is fine for kids (warning: I have not tried this specifically on primary school, so the patience required might be too high):
Modular arithmetic modulo small numbers is required, but kids are more than happy to accept it. Write on a piece of paper that (only on this piece of paper) 3 = 0. Then talk about it; if 3 = 0, then for sure also 4 = 1. And 11 = 8 = 5 = 2. When they have to do 2 + 2, they get 4, but you say "wait a minute, on this paper, what is a simpler way to write 4?" and you get 2 + 2 = 1 mod 3.
Then you build Pascal's Triangle modulo 3, or modulo 4, or whatever modulus they want to try. The algorithm for building the next row is easy, and the patterns you get out of this are pretty awesome. For some visuals in advance, check out https://www.maa.org/press/periodicals/loci/joma/patterns-in-pascals-triangle-with-a-twist-first-twist-what-is-it . You can get these patterns by just not writing the 0's in Pascal's triangle, or by writing each number in a different color.
For maximum prettiness of the pictures, you will want to use hexagonal grid paper with very light lines so that kids can perfectly space the numbers. At the time of this post, https://incompetech.com/graphpaper/hexagonal/ still lets you generate free hexagonal graph paper and choose the weight of the lines. If too much time has passed, hopefully some other kind internet soul has some for you.