I spent some time doing LEGO math with my 4-year-old today. Phase 1, numbers - build stacks for each number 1-9 and put them in order. Phase 2, addition - combine stacks and see what they make. Phase 3, subtraction - take away bricks from a stack.
BTW, that's 1-year-old's hand just behind her brother in the second picture, and that's her artwork on the wall. Each kid has gone through this write-on-the-wall stage. Time to paint the kitchen again. :(